[mirror_ubuntu-hirsute-kernel.git] / arch / x86 / kernel / entry_64.S

/*
 *  linux/arch/x86_64/entry.S
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *  Copyright (C) 2000, 2001, 2002  Andi Kleen SuSE Labs
 *  Copyright (C) 2000  Pavel Machek <pavel@suse.cz>
 */

/*
 * entry.S contains the system-call and fault low-level handling routines.
 *
 * Some of this is documented in Documentation/x86/entry_64.txt
 *
 * NOTE: This code handles signal-recognition, which happens every time
 * after an interrupt and after each system call.
 *
 * A note on terminology:
 * - iret frame: Architecture defined interrupt frame from SS to RIP
 * at the top of the kernel process stack.
 *
 * Some macro usage:
 * - CFI macros are used to generate dwarf2 unwind information for better
 * backtraces. They don't change any code.
 * - ENTRY/END Define functions in the symbol table.
 * - TRACE_IRQ_* - Trace hard interrupt state for lock debugging.
 * - idtentry - Define exception entry points.
 */

#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/cache.h>
#include <asm/errno.h>
#include <asm/dwarf2.h>
#include <asm/calling.h>
#include <asm/asm-offsets.h>
#include <asm/msr.h>
#include <asm/unistd.h>
#include <asm/thread_info.h>
#include <asm/hw_irq.h>
#include <asm/page_types.h>
#include <asm/irqflags.h>
#include <asm/paravirt.h>
#include <asm/percpu.h>
#include <asm/asm.h>
#include <asm/context_tracking.h>
#include <asm/smap.h>
#include <asm/pgtable_types.h>
#include <linux/err.h>

/* Avoid __ASSEMBLER__'ifying <linux/audit.h> just for this.  */
#include <linux/elf-em.h>
#define AUDIT_ARCH_X86_64       (EM_X86_64|__AUDIT_ARCH_64BIT|__AUDIT_ARCH_LE)
#define __AUDIT_ARCH_64BIT 0x80000000
#define __AUDIT_ARCH_LE    0x40000000

        .code64
        .section .entry.text, "ax"


#ifdef CONFIG_PARAVIRT
ENTRY(native_usergs_sysret64)
        swapgs
        sysretq
ENDPROC(native_usergs_sysret64)
#endif /* CONFIG_PARAVIRT */


.macro TRACE_IRQS_IRETQ
#ifdef CONFIG_TRACE_IRQFLAGS
        bt   $9,EFLAGS(%rsp)    /* interrupts off? */
        jnc  1f
        TRACE_IRQS_ON
1:
#endif
.endm

/*
 * When dynamic function tracer is enabled it will add a breakpoint
 * to all locations that it is about to modify, sync CPUs, update
 * all the code, sync CPUs, then remove the breakpoints. In this time
 * if lockdep is enabled, it might jump back into the debug handler
 * outside the updating of the IST protection. (TRACE_IRQS_ON/OFF).
 *
 * We need to change the IDT table before calling TRACE_IRQS_ON/OFF to
 * make sure the stack pointer does not get reset back to the top
 * of the debug stack, and instead just reuses the current stack.
 */
#if defined(CONFIG_DYNAMIC_FTRACE) && defined(CONFIG_TRACE_IRQFLAGS)

.macro TRACE_IRQS_OFF_DEBUG
        call debug_stack_set_zero
        TRACE_IRQS_OFF
        call debug_stack_reset
.endm

.macro TRACE_IRQS_ON_DEBUG
        call debug_stack_set_zero
        TRACE_IRQS_ON
        call debug_stack_reset
.endm

.macro TRACE_IRQS_IRETQ_DEBUG
        bt   $9,EFLAGS(%rsp)    /* interrupts off? */
        jnc  1f
        TRACE_IRQS_ON_DEBUG
1:
.endm

#else
# define TRACE_IRQS_OFF_DEBUG           TRACE_IRQS_OFF
# define TRACE_IRQS_ON_DEBUG            TRACE_IRQS_ON
# define TRACE_IRQS_IRETQ_DEBUG         TRACE_IRQS_IRETQ
#endif

/*
 * empty frame
 */
        .macro EMPTY_FRAME start=1 offset=0
        .if \start
        CFI_STARTPROC simple
        CFI_SIGNAL_FRAME
        CFI_DEF_CFA rsp,8+\offset
        .else
        CFI_DEF_CFA_OFFSET 8+\offset
        .endif
        .endm

/*
 * initial frame state for interrupts (and exceptions without error code)
 */
        .macro INTR_FRAME start=1 offset=0
        EMPTY_FRAME \start, 5*8+\offset
        /*CFI_REL_OFFSET ss, 4*8+\offset*/
        CFI_REL_OFFSET rsp, 3*8+\offset
        /*CFI_REL_OFFSET rflags, 2*8+\offset*/
        /*CFI_REL_OFFSET cs, 1*8+\offset*/
        CFI_REL_OFFSET rip, 0*8+\offset
        .endm

/*
 * initial frame state for exceptions with error code (and interrupts
 * with vector already pushed)
 */
        .macro XCPT_FRAME start=1 offset=0
        INTR_FRAME \start, 1*8+\offset
        .endm

/*
 * frame that enables passing a complete pt_regs to a C function.
 */
        .macro DEFAULT_FRAME start=1 offset=0
        XCPT_FRAME \start, ORIG_RAX+\offset
        CFI_REL_OFFSET rdi, RDI+\offset
        CFI_REL_OFFSET rsi, RSI+\offset
        CFI_REL_OFFSET rdx, RDX+\offset
        CFI_REL_OFFSET rcx, RCX+\offset
        CFI_REL_OFFSET rax, RAX+\offset
        CFI_REL_OFFSET r8, R8+\offset
        CFI_REL_OFFSET r9, R9+\offset
        CFI_REL_OFFSET r10, R10+\offset
        CFI_REL_OFFSET r11, R11+\offset
        CFI_REL_OFFSET rbx, RBX+\offset
        CFI_REL_OFFSET rbp, RBP+\offset
        CFI_REL_OFFSET r12, R12+\offset
        CFI_REL_OFFSET r13, R13+\offset
        CFI_REL_OFFSET r14, R14+\offset
        CFI_REL_OFFSET r15, R15+\offset
        .endm

/*
 * 64bit SYSCALL instruction entry. Up to 6 arguments in registers.
 *
 * 64bit SYSCALL saves rip to rcx, clears rflags.RF, then saves rflags to r11,
 * then loads new ss, cs, and rip from previously programmed MSRs.
 * rflags gets masked by a value from another MSR (so CLD and CLAC
 * are not needed). SYSCALL does not save anything on the stack
 * and does not change rsp.
 *
 * Registers on entry:
 * rax  system call number
 * rcx  return address
 * r11  saved rflags (note: r11 is callee-clobbered register in C ABI)
 * rdi  arg0
 * rsi  arg1
 * rdx  arg2
 * r10  arg3 (needs to be moved to rcx to conform to C ABI)
 * r8   arg4
 * r9   arg5
 * (note: r12-r15,rbp,rbx are callee-preserved in C ABI)
 *
 * Only called from user space.
 *
 * When user can change pt_regs->foo always force IRET. That is because
 * it deals with uncanonical addresses better. SYSRET has trouble
 * with them due to bugs in both AMD and Intel CPUs.
 */

ENTRY(system_call)
        CFI_STARTPROC   simple
        CFI_SIGNAL_FRAME
        CFI_DEF_CFA     rsp,0
        CFI_REGISTER    rip,rcx
        /*CFI_REGISTER  rflags,r11*/

        /*
         * Interrupts are off on entry.
         * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
         * it is too small to ever cause noticeable irq latency.
         */
        SWAPGS_UNSAFE_STACK
        /*
         * A hypervisor implementation might want to use a label
         * after the swapgs, so that it can do the swapgs
         * for the guest and jump here on syscall.
         */
GLOBAL(system_call_after_swapgs)

        movq    %rsp,PER_CPU_VAR(rsp_scratch)
        movq    PER_CPU_VAR(kernel_stack),%rsp

        /* Construct struct pt_regs on stack */
        pushq_cfi $__USER_DS                    /* pt_regs->ss */
        pushq_cfi PER_CPU_VAR(rsp_scratch)      /* pt_regs->sp */
        /*
         * Re-enable interrupts.
         * We use 'rsp_scratch' as a scratch space, hence irq-off block above
         * must execute atomically in the face of possible interrupt-driven
         * task preemption. We must enable interrupts only after we're done
         * with using rsp_scratch:
         */
        ENABLE_INTERRUPTS(CLBR_NONE)
        pushq_cfi       %r11                    /* pt_regs->flags */
        pushq_cfi       $__USER_CS              /* pt_regs->cs */
        pushq_cfi       %rcx                    /* pt_regs->ip */
        CFI_REL_OFFSET rip,0
        pushq_cfi_reg   rax                     /* pt_regs->orig_ax */
        pushq_cfi_reg   rdi                     /* pt_regs->di */
        pushq_cfi_reg   rsi                     /* pt_regs->si */
        pushq_cfi_reg   rdx                     /* pt_regs->dx */
        pushq_cfi_reg   rcx                     /* pt_regs->cx */
        pushq_cfi       $-ENOSYS                /* pt_regs->ax */
        pushq_cfi_reg   r8                      /* pt_regs->r8 */
        pushq_cfi_reg   r9                      /* pt_regs->r9 */
        pushq_cfi_reg   r10                     /* pt_regs->r10 */
        pushq_cfi_reg   r11                     /* pt_regs->r11 */
        sub     $(6*8),%rsp /* pt_regs->bp,bx,r12-15 not saved */
        CFI_ADJUST_CFA_OFFSET 6*8

        testl $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
        jnz tracesys
system_call_fastpath:
#if __SYSCALL_MASK == ~0
        cmpq $__NR_syscall_max,%rax
#else
        andl $__SYSCALL_MASK,%eax
        cmpl $__NR_syscall_max,%eax
#endif
        ja      1f      /* return -ENOSYS (already in pt_regs->ax) */
        movq %r10,%rcx
        call *sys_call_table(,%rax,8)
        movq %rax,RAX(%rsp)
1:
/*
 * Syscall return path ending with SYSRET (fast path).
 * Has incompletely filled pt_regs.
 */
        LOCKDEP_SYS_EXIT
        /*
         * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON,
         * it is too small to ever cause noticeable irq latency.
         */
        DISABLE_INTERRUPTS(CLBR_NONE)

        /*
         * We must check ti flags with interrupts (or at least preemption)
         * off because we must *never* return to userspace without
         * processing exit work that is enqueued if we're preempted here.
         * In particular, returning to userspace with any of the one-shot
         * flags (TIF_NOTIFY_RESUME, TIF_USER_RETURN_NOTIFY, etc) set is
         * very bad.
         */
        testl $_TIF_ALLWORK_MASK, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
        jnz int_ret_from_sys_call_irqs_off      /* Go to the slow path */

        CFI_REMEMBER_STATE

        RESTORE_C_REGS_EXCEPT_RCX_R11
        movq    RIP(%rsp),%rcx
        CFI_REGISTER    rip,rcx
        movq    EFLAGS(%rsp),%r11
        /*CFI_REGISTER  rflags,r11*/
        movq    RSP(%rsp),%rsp
        /*
         * 64bit SYSRET restores rip from rcx,
         * rflags from r11 (but RF and VM bits are forced to 0),
         * cs and ss are loaded from MSRs.
         * Restoration of rflags re-enables interrupts.
         */
        USERGS_SYSRET64

        CFI_RESTORE_STATE

        /* Do syscall entry tracing */
tracesys:
        movq %rsp, %rdi
        movl $AUDIT_ARCH_X86_64, %esi
        call syscall_trace_enter_phase1
        test %rax, %rax
        jnz tracesys_phase2             /* if needed, run the slow path */
        RESTORE_C_REGS_EXCEPT_RAX       /* else restore clobbered regs */
        movq ORIG_RAX(%rsp), %rax
        jmp system_call_fastpath        /*      and return to the fast path */

tracesys_phase2:
        SAVE_EXTRA_REGS
        movq %rsp, %rdi
        movl $AUDIT_ARCH_X86_64, %esi
        movq %rax,%rdx
        call syscall_trace_enter_phase2

        /*
         * Reload registers from stack in case ptrace changed them.
         * We don't reload %rax because syscall_trace_entry_phase2() returned
         * the value it wants us to use in the table lookup.
         */
        RESTORE_C_REGS_EXCEPT_RAX
        RESTORE_EXTRA_REGS
#if __SYSCALL_MASK == ~0
        cmpq $__NR_syscall_max,%rax
#else
        andl $__SYSCALL_MASK,%eax
        cmpl $__NR_syscall_max,%eax
#endif
        ja      1f      /* return -ENOSYS (already in pt_regs->ax) */
        movq %r10,%rcx  /* fixup for C */
        call *sys_call_table(,%rax,8)
        movq %rax,RAX(%rsp)
1:
        /* Use IRET because user could have changed pt_regs->foo */

/*
 * Syscall return path ending with IRET.
 * Has correct iret frame.
 */
GLOBAL(int_ret_from_sys_call)
        DISABLE_INTERRUPTS(CLBR_NONE)
int_ret_from_sys_call_irqs_off: /* jumps come here from the irqs-off SYSRET path */
        TRACE_IRQS_OFF
        movl $_TIF_ALLWORK_MASK,%edi
        /* edi: mask to check */
GLOBAL(int_with_check)
        LOCKDEP_SYS_EXIT_IRQ
        GET_THREAD_INFO(%rcx)
        movl TI_flags(%rcx),%edx
        andl %edi,%edx
        jnz   int_careful
        andl    $~TS_COMPAT,TI_status(%rcx)
        jmp     syscall_return

        /* Either reschedule or signal or syscall exit tracking needed. */
        /* First do a reschedule test. */
        /* edx: work, edi: workmask */
int_careful:
        bt $TIF_NEED_RESCHED,%edx
        jnc  int_very_careful
        TRACE_IRQS_ON
        ENABLE_INTERRUPTS(CLBR_NONE)
        pushq_cfi %rdi
        SCHEDULE_USER
        popq_cfi %rdi
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        jmp int_with_check

        /* handle signals and tracing -- both require a full pt_regs */
int_very_careful:
        TRACE_IRQS_ON
        ENABLE_INTERRUPTS(CLBR_NONE)
        SAVE_EXTRA_REGS
        /* Check for syscall exit trace */
        testl $_TIF_WORK_SYSCALL_EXIT,%edx
        jz int_signal
        pushq_cfi %rdi
        leaq 8(%rsp),%rdi       # &ptregs -> arg1
        call syscall_trace_leave
        popq_cfi %rdi
        andl $~(_TIF_WORK_SYSCALL_EXIT|_TIF_SYSCALL_EMU),%edi
        jmp int_restore_rest

int_signal:
        testl $_TIF_DO_NOTIFY_MASK,%edx
        jz 1f
        movq %rsp,%rdi          # &ptregs -> arg1
        xorl %esi,%esi          # oldset -> arg2
        call do_notify_resume
1:      movl $_TIF_WORK_MASK,%edi
int_restore_rest:
        RESTORE_EXTRA_REGS
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        jmp int_with_check

syscall_return:
        /* The IRETQ could re-enable interrupts: */
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_IRETQ

        /*
         * Try to use SYSRET instead of IRET if we're returning to
         * a completely clean 64-bit userspace context.
         */
        movq RCX(%rsp),%rcx
        cmpq %rcx,RIP(%rsp)             /* RCX == RIP */
        jne opportunistic_sysret_failed

        /*
         * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
         * in kernel space.  This essentially lets the user take over
         * the kernel, since userspace controls RSP.  It's not worth
         * testing for canonicalness exactly -- this check detects any
         * of the 17 high bits set, which is true for non-canonical
         * or kernel addresses.  (This will pessimize vsyscall=native.
         * Big deal.)
         *
         * If virtual addresses ever become wider, this will need
         * to be updated to remain correct on both old and new CPUs.
         */
        .ifne __VIRTUAL_MASK_SHIFT - 47
        .error "virtual address width changed -- SYSRET checks need update"
        .endif
        shr $__VIRTUAL_MASK_SHIFT, %rcx
        jnz opportunistic_sysret_failed

        cmpq $__USER_CS,CS(%rsp)        /* CS must match SYSRET */
        jne opportunistic_sysret_failed

        movq R11(%rsp),%r11
        cmpq %r11,EFLAGS(%rsp)          /* R11 == RFLAGS */
        jne opportunistic_sysret_failed

        /*
         * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
         * restoring TF results in a trap from userspace immediately after
         * SYSRET.  This would cause an infinite loop whenever #DB happens
         * with register state that satisfies the opportunistic SYSRET
         * conditions.  For example, single-stepping this user code:
         *
         *           movq $stuck_here,%rcx
         *           pushfq
         *           popq %r11
         *   stuck_here:
         *
         * would never get past 'stuck_here'.
         */
        testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
        jnz opportunistic_sysret_failed

        /* nothing to check for RSP */

        cmpq $__USER_DS,SS(%rsp)        /* SS must match SYSRET */
        jne opportunistic_sysret_failed

        /*
         * We win!  This label is here just for ease of understanding
         * perf profiles.  Nothing jumps here.
         */
syscall_return_via_sysret:
        CFI_REMEMBER_STATE
        /* r11 is already restored (see code above) */
        RESTORE_C_REGS_EXCEPT_R11
        movq RSP(%rsp),%rsp
        USERGS_SYSRET64
        CFI_RESTORE_STATE

opportunistic_sysret_failed:
        SWAPGS
        jmp     restore_c_regs_and_iret
        CFI_ENDPROC
END(system_call)


        .macro FORK_LIKE func
ENTRY(stub_\func)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8              /* offset 8: return address */
        SAVE_EXTRA_REGS 8
        jmp sys_\func
        CFI_ENDPROC
END(stub_\func)
        .endm

        FORK_LIKE  clone
        FORK_LIKE  fork
        FORK_LIKE  vfork

ENTRY(stub_execve)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        call    sys_execve
return_from_execve:
        testl   %eax, %eax
        jz      1f
        /* exec failed, can use fast SYSRET code path in this case */
        ret
1:
        /* must use IRET code path (pt_regs->cs may have changed) */
        addq    $8, %rsp
        CFI_ADJUST_CFA_OFFSET -8
        ZERO_EXTRA_REGS
        movq    %rax,RAX(%rsp)
        jmp     int_ret_from_sys_call
        CFI_ENDPROC
END(stub_execve)

ENTRY(stub_execveat)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        call    sys_execveat
        jmp     return_from_execve
        CFI_ENDPROC
END(stub_execveat)

#ifdef CONFIG_X86_X32_ABI
ENTRY(stub_x32_execve)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        call    compat_sys_execve
        jmp     return_from_execve
        CFI_ENDPROC
END(stub_x32_execve)
ENTRY(stub_x32_execveat)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        call    compat_sys_execveat
        jmp     return_from_execve
        CFI_ENDPROC
END(stub_x32_execveat)
#endif

#ifdef CONFIG_IA32_EMULATION
ENTRY(stub32_execve)
        CFI_STARTPROC
        call    compat_sys_execve
        jmp     return_from_execve
        CFI_ENDPROC
END(stub32_execve)
ENTRY(stub32_execveat)
        CFI_STARTPROC
        call    compat_sys_execveat
        jmp     return_from_execve
        CFI_ENDPROC
END(stub32_execveat)
#endif

/*
 * sigreturn is special because it needs to restore all registers on return.
 * This cannot be done with SYSRET, so use the IRET return path instead.
 */
ENTRY(stub_rt_sigreturn)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        /*
         * SAVE_EXTRA_REGS result is not normally needed:
         * sigreturn overwrites all pt_regs->GPREGS.
         * But sigreturn can fail (!), and there is no easy way to detect that.
         * To make sure RESTORE_EXTRA_REGS doesn't restore garbage on error,
         * we SAVE_EXTRA_REGS here.
         */
        SAVE_EXTRA_REGS 8
        call sys_rt_sigreturn
return_from_stub:
        addq    $8, %rsp
        CFI_ADJUST_CFA_OFFSET -8
        RESTORE_EXTRA_REGS
        movq %rax,RAX(%rsp)
        jmp int_ret_from_sys_call
        CFI_ENDPROC
END(stub_rt_sigreturn)

#ifdef CONFIG_X86_X32_ABI
ENTRY(stub_x32_rt_sigreturn)
        CFI_STARTPROC
        DEFAULT_FRAME 0, 8
        SAVE_EXTRA_REGS 8
        call sys32_x32_rt_sigreturn
        jmp  return_from_stub
        CFI_ENDPROC
END(stub_x32_rt_sigreturn)
#endif

/*
 * A newly forked process directly context switches into this address.
 *
 * rdi: prev task we switched from
 */
ENTRY(ret_from_fork)
        DEFAULT_FRAME

        LOCK ; btr $TIF_FORK,TI_flags(%r8)

        pushq_cfi $0x0002
        popfq_cfi                               # reset kernel eflags

        call schedule_tail                      # rdi: 'prev' task parameter

        GET_THREAD_INFO(%rcx)

        RESTORE_EXTRA_REGS

        testl $3,CS(%rsp)                       # from kernel_thread?
        jz   1f

        /*
         * By the time we get here, we have no idea whether our pt_regs,
         * ti flags, and ti status came from the 64-bit SYSCALL fast path,
         * the slow path, or one of the ia32entry paths.
         * Use int_ret_from_sys_call to return, since it can safely handle
         * all of the above.
         */
        jmp  int_ret_from_sys_call

1:
        movq %rbp, %rdi
        call *%rbx
        movl $0, RAX(%rsp)
        RESTORE_EXTRA_REGS
        jmp int_ret_from_sys_call
        CFI_ENDPROC
END(ret_from_fork)

/*
 * Build the entry stubs with some assembler magic.
 * We pack 1 stub into every 8-byte block.
 */
        .align 8
ENTRY(irq_entries_start)
        INTR_FRAME
    vector=FIRST_EXTERNAL_VECTOR
    .rept (FIRST_SYSTEM_VECTOR - FIRST_EXTERNAL_VECTOR)
        pushq_cfi $(~vector+0x80)       /* Note: always in signed byte range */
    vector=vector+1
        jmp     common_interrupt
        CFI_ADJUST_CFA_OFFSET -8
        .align  8
    .endr
        CFI_ENDPROC
END(irq_entries_start)

/*
 * Interrupt entry/exit.
 *
 * Interrupt entry points save only callee clobbered registers in fast path.
 *
 * Entry runs with interrupts off.
 */

/* 0(%rsp): ~(interrupt number) */
        .macro interrupt func
        cld
        /*
         * Since nothing in interrupt handling code touches r12...r15 members
         * of "struct pt_regs", and since interrupts can nest, we can save
         * four stack slots and simultaneously provide
         * an unwind-friendly stack layout by saving "truncated" pt_regs
         * exactly up to rbp slot, without these members.
         */
        ALLOC_PT_GPREGS_ON_STACK -RBP
        SAVE_C_REGS -RBP
        /* this goes to 0(%rsp) for unwinder, not for saving the value: */
        SAVE_EXTRA_REGS_RBP -RBP

        leaq -RBP(%rsp),%rdi    /* arg1 for \func (pointer to pt_regs) */

        testl $3, CS-RBP(%rsp)
        je 1f
        SWAPGS
1:
        /*
         * Save previous stack pointer, optionally switch to interrupt stack.
         * irq_count is used to check if a CPU is already on an interrupt stack
         * or not. While this is essentially redundant with preempt_count it is
         * a little cheaper to use a separate counter in the PDA (short of
         * moving irq_enter into assembly, which would be too much work)
         */
        movq %rsp, %rsi
        incl PER_CPU_VAR(irq_count)
        cmovzq PER_CPU_VAR(irq_stack_ptr),%rsp
        CFI_DEF_CFA_REGISTER    rsi
        pushq %rsi
        /*
         * For debugger:
         * "CFA (Current Frame Address) is the value on stack + offset"
         */
        CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
                        0x77 /* DW_OP_breg7 (rsp) */, 0, \
                        0x06 /* DW_OP_deref */, \
                        0x08 /* DW_OP_const1u */, SIZEOF_PTREGS-RBP, \
                        0x22 /* DW_OP_plus */
        /* We entered an interrupt context - irqs are off: */
        TRACE_IRQS_OFF

        call \func
        .endm

        /*
         * The interrupt stubs push (~vector+0x80) onto the stack and
         * then jump to common_interrupt.
         */
        .p2align CONFIG_X86_L1_CACHE_SHIFT
common_interrupt:
        XCPT_FRAME
        ASM_CLAC
        addq $-0x80,(%rsp)              /* Adjust vector to [-256,-1] range */
        interrupt do_IRQ
        /* 0(%rsp): old RSP */
ret_from_intr:
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        decl PER_CPU_VAR(irq_count)

        /* Restore saved previous stack */
        popq %rsi
        CFI_DEF_CFA rsi,SIZEOF_PTREGS-RBP /* reg/off reset after def_cfa_expr */
        /* return code expects complete pt_regs - adjust rsp accordingly: */
        leaq -RBP(%rsi),%rsp
        CFI_DEF_CFA_REGISTER    rsp
        CFI_ADJUST_CFA_OFFSET   RBP

        testl $3,CS(%rsp)
        je retint_kernel
        /* Interrupt came from user space */

        GET_THREAD_INFO(%rcx)
        /*
         * %rcx: thread info. Interrupts off.
         */
retint_with_reschedule:
        movl $_TIF_WORK_MASK,%edi
retint_check:
        LOCKDEP_SYS_EXIT_IRQ
        movl TI_flags(%rcx),%edx
        andl %edi,%edx
        CFI_REMEMBER_STATE
        jnz  retint_careful

retint_swapgs:          /* return to user-space */
        /*
         * The iretq could re-enable interrupts:
         */
        DISABLE_INTERRUPTS(CLBR_ANY)
        TRACE_IRQS_IRETQ

        SWAPGS
        jmp     restore_c_regs_and_iret

/* Returning to kernel space */
retint_kernel:
#ifdef CONFIG_PREEMPT
        /* Interrupts are off */
        /* Check if we need preemption */
        bt      $9,EFLAGS(%rsp) /* interrupts were off? */
        jnc     1f
0:      cmpl    $0,PER_CPU_VAR(__preempt_count)
        jnz     1f
        call    preempt_schedule_irq
        jmp     0b
1:
#endif
        /*
         * The iretq could re-enable interrupts:
         */
        TRACE_IRQS_IRETQ

/*
 * At this label, code paths which return to kernel and to user,
 * which come from interrupts/exception and from syscalls, merge.
 */
restore_c_regs_and_iret:
        RESTORE_C_REGS
        REMOVE_PT_GPREGS_FROM_STACK 8

irq_return:
        INTERRUPT_RETURN

ENTRY(native_iret)
        /*
         * Are we returning to a stack segment from the LDT?  Note: in
         * 64-bit mode SS:RSP on the exception stack is always valid.
         */
#ifdef CONFIG_X86_ESPFIX64
        testb $4,(SS-RIP)(%rsp)
        jnz native_irq_return_ldt
#endif

.global native_irq_return_iret
native_irq_return_iret:
        /*
         * This may fault.  Non-paranoid faults on return to userspace are
         * handled by fixup_bad_iret.  These include #SS, #GP, and #NP.
         * Double-faults due to espfix64 are handled in do_double_fault.
         * Other faults here are fatal.
         */
        iretq

#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
        pushq_cfi %rax
        pushq_cfi %rdi
        SWAPGS
        movq PER_CPU_VAR(espfix_waddr),%rdi
        movq %rax,(0*8)(%rdi)   /* RAX */
        movq (2*8)(%rsp),%rax   /* RIP */
        movq %rax,(1*8)(%rdi)
        movq (3*8)(%rsp),%rax   /* CS */
        movq %rax,(2*8)(%rdi)
        movq (4*8)(%rsp),%rax   /* RFLAGS */
        movq %rax,(3*8)(%rdi)
        movq (6*8)(%rsp),%rax   /* SS */
        movq %rax,(5*8)(%rdi)
        movq (5*8)(%rsp),%rax   /* RSP */
        movq %rax,(4*8)(%rdi)
        andl $0xffff0000,%eax
        popq_cfi %rdi
        orq PER_CPU_VAR(espfix_stack),%rax
        SWAPGS
        movq %rax,%rsp
        popq_cfi %rax
        jmp native_irq_return_iret
#endif

        /* edi: workmask, edx: work */
retint_careful:
        CFI_RESTORE_STATE
        bt    $TIF_NEED_RESCHED,%edx
        jnc   retint_signal
        TRACE_IRQS_ON
        ENABLE_INTERRUPTS(CLBR_NONE)
        pushq_cfi %rdi
        SCHEDULE_USER
        popq_cfi %rdi
        GET_THREAD_INFO(%rcx)
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        jmp retint_check

retint_signal:
        testl $_TIF_DO_NOTIFY_MASK,%edx
        jz    retint_swapgs
        TRACE_IRQS_ON
        ENABLE_INTERRUPTS(CLBR_NONE)
        SAVE_EXTRA_REGS
        movq $-1,ORIG_RAX(%rsp)
        xorl %esi,%esi          # oldset
        movq %rsp,%rdi          # &pt_regs
        call do_notify_resume
        RESTORE_EXTRA_REGS
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        GET_THREAD_INFO(%rcx)
        jmp retint_with_reschedule

        CFI_ENDPROC
END(common_interrupt)

/*
 * APIC interrupts.
 */
.macro apicinterrupt3 num sym do_sym
ENTRY(\sym)
        INTR_FRAME
        ASM_CLAC
        pushq_cfi $~(\num)
.Lcommon_\sym:
        interrupt \do_sym
        jmp ret_from_intr
        CFI_ENDPROC
END(\sym)
.endm

#ifdef CONFIG_TRACING
#define trace(sym) trace_##sym
#define smp_trace(sym) smp_trace_##sym

.macro trace_apicinterrupt num sym
apicinterrupt3 \num trace(\sym) smp_trace(\sym)
.endm
#else
.macro trace_apicinterrupt num sym do_sym
.endm
#endif

.macro apicinterrupt num sym do_sym
apicinterrupt3 \num \sym \do_sym
trace_apicinterrupt \num \sym
.endm

#ifdef CONFIG_SMP
apicinterrupt3 IRQ_MOVE_CLEANUP_VECTOR \
        irq_move_cleanup_interrupt smp_irq_move_cleanup_interrupt
apicinterrupt3 REBOOT_VECTOR \
        reboot_interrupt smp_reboot_interrupt
#endif

#ifdef CONFIG_X86_UV
apicinterrupt3 UV_BAU_MESSAGE \
        uv_bau_message_intr1 uv_bau_message_interrupt
#endif
apicinterrupt LOCAL_TIMER_VECTOR \
        apic_timer_interrupt smp_apic_timer_interrupt
apicinterrupt X86_PLATFORM_IPI_VECTOR \
        x86_platform_ipi smp_x86_platform_ipi

#ifdef CONFIG_HAVE_KVM
apicinterrupt3 POSTED_INTR_VECTOR \
        kvm_posted_intr_ipi smp_kvm_posted_intr_ipi
#endif

#ifdef CONFIG_X86_MCE_THRESHOLD
apicinterrupt THRESHOLD_APIC_VECTOR \
        threshold_interrupt smp_threshold_interrupt
#endif

#ifdef CONFIG_X86_THERMAL_VECTOR
apicinterrupt THERMAL_APIC_VECTOR \
        thermal_interrupt smp_thermal_interrupt
#endif

#ifdef CONFIG_SMP
apicinterrupt CALL_FUNCTION_SINGLE_VECTOR \
        call_function_single_interrupt smp_call_function_single_interrupt
apicinterrupt CALL_FUNCTION_VECTOR \
        call_function_interrupt smp_call_function_interrupt
apicinterrupt RESCHEDULE_VECTOR \
        reschedule_interrupt smp_reschedule_interrupt
#endif

apicinterrupt ERROR_APIC_VECTOR \
        error_interrupt smp_error_interrupt
apicinterrupt SPURIOUS_APIC_VECTOR \
        spurious_interrupt smp_spurious_interrupt

#ifdef CONFIG_IRQ_WORK
apicinterrupt IRQ_WORK_VECTOR \
        irq_work_interrupt smp_irq_work_interrupt
#endif

/*
 * Exception entry points.
 */
#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8)

.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
ENTRY(\sym)
        /* Sanity check */
        .if \shift_ist != -1 && \paranoid == 0
        .error "using shift_ist requires paranoid=1"
        .endif

        .if \has_error_code
        XCPT_FRAME
        .else
        INTR_FRAME
        .endif

        ASM_CLAC
        PARAVIRT_ADJUST_EXCEPTION_FRAME

        .ifeq \has_error_code
        pushq_cfi $-1                   /* ORIG_RAX: no syscall to restart */
        .endif

        ALLOC_PT_GPREGS_ON_STACK

        .if \paranoid
        .if \paranoid == 1
        CFI_REMEMBER_STATE
        testl $3, CS(%rsp)              /* If coming from userspace, switch */
        jnz 1f                          /* stacks. */
        .endif
        call paranoid_entry
        .else
        call error_entry
        .endif
        /* returned flag: ebx=0: need swapgs on exit, ebx=1: don't need it */

        DEFAULT_FRAME 0

        .if \paranoid
        .if \shift_ist != -1
        TRACE_IRQS_OFF_DEBUG            /* reload IDT in case of recursion */
        .else
        TRACE_IRQS_OFF
        .endif
        .endif

        movq %rsp,%rdi                  /* pt_regs pointer */

        .if \has_error_code
        movq ORIG_RAX(%rsp),%rsi        /* get error code */
        movq $-1,ORIG_RAX(%rsp)         /* no syscall to restart */
        .else
        xorl %esi,%esi                  /* no error code */
        .endif

        .if \shift_ist != -1
        subq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
        .endif

        call \do_sym

        .if \shift_ist != -1
        addq $EXCEPTION_STKSZ, CPU_TSS_IST(\shift_ist)
        .endif

        /* these procedures expect "no swapgs" flag in ebx */
        .if \paranoid
        jmp paranoid_exit
        .else
        jmp error_exit
        .endif

        .if \paranoid == 1
        CFI_RESTORE_STATE
        /*
         * Paranoid entry from userspace.  Switch stacks and treat it
         * as a normal entry.  This means that paranoid handlers
         * run in real process context if user_mode(regs).
         */
1:
        call error_entry

        DEFAULT_FRAME 0

        movq %rsp,%rdi                  /* pt_regs pointer */
        call sync_regs
        movq %rax,%rsp                  /* switch stack */

        movq %rsp,%rdi                  /* pt_regs pointer */

        .if \has_error_code
        movq ORIG_RAX(%rsp),%rsi        /* get error code */
        movq $-1,ORIG_RAX(%rsp)         /* no syscall to restart */
        .else
        xorl %esi,%esi                  /* no error code */
        .endif

        call \do_sym

        jmp error_exit                  /* %ebx: no swapgs flag */
        .endif

        CFI_ENDPROC
END(\sym)
.endm

#ifdef CONFIG_TRACING
.macro trace_idtentry sym do_sym has_error_code:req
idtentry trace(\sym) trace(\do_sym) has_error_code=\has_error_code
idtentry \sym \do_sym has_error_code=\has_error_code
.endm
#else
.macro trace_idtentry sym do_sym has_error_code:req
idtentry \sym \do_sym has_error_code=\has_error_code
.endm
#endif

idtentry divide_error do_divide_error has_error_code=0
idtentry overflow do_overflow has_error_code=0
idtentry bounds do_bounds has_error_code=0
idtentry invalid_op do_invalid_op has_error_code=0
idtentry device_not_available do_device_not_available has_error_code=0
idtentry double_fault do_double_fault has_error_code=1 paranoid=2
idtentry coprocessor_segment_overrun do_coprocessor_segment_overrun has_error_code=0
idtentry invalid_TSS do_invalid_TSS has_error_code=1
idtentry segment_not_present do_segment_not_present has_error_code=1
idtentry spurious_interrupt_bug do_spurious_interrupt_bug has_error_code=0
idtentry coprocessor_error do_coprocessor_error has_error_code=0
idtentry alignment_check do_alignment_check has_error_code=1
idtentry simd_coprocessor_error do_simd_coprocessor_error has_error_code=0


        /* Reload gs selector with exception handling */
        /* edi:  new selector */
ENTRY(native_load_gs_index)
        CFI_STARTPROC
        pushfq_cfi
        DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
        SWAPGS
gs_change:
        movl %edi,%gs
2:      mfence          /* workaround */
        SWAPGS
        popfq_cfi
        ret
        CFI_ENDPROC
END(native_load_gs_index)

        _ASM_EXTABLE(gs_change,bad_gs)
        .section .fixup,"ax"
        /* running with kernelgs */
bad_gs:
        SWAPGS                  /* switch back to user gs */
        xorl %eax,%eax
        movl %eax,%gs
        jmp  2b
        .previous

/* Call softirq on interrupt stack. Interrupts are off. */
ENTRY(do_softirq_own_stack)
        CFI_STARTPROC
        pushq_cfi %rbp
        CFI_REL_OFFSET rbp,0
        mov  %rsp,%rbp
        CFI_DEF_CFA_REGISTER rbp
        incl PER_CPU_VAR(irq_count)
        cmove PER_CPU_VAR(irq_stack_ptr),%rsp
        push  %rbp                      # backlink for old unwinder
        call __do_softirq
        leaveq
        CFI_RESTORE             rbp
        CFI_DEF_CFA_REGISTER    rsp
        CFI_ADJUST_CFA_OFFSET   -8
        decl PER_CPU_VAR(irq_count)
        ret
        CFI_ENDPROC
END(do_softirq_own_stack)

#ifdef CONFIG_XEN
idtentry xen_hypervisor_callback xen_do_hypervisor_callback has_error_code=0

/*
 * A note on the "critical region" in our callback handler.
 * We want to avoid stacking callback handlers due to events occurring
 * during handling of the last event. To do this, we keep events disabled
 * until we've done all processing. HOWEVER, we must enable events before
 * popping the stack frame (can't be done atomically) and so it would still
 * be possible to get enough handler activations to overflow the stack.
 * Although unlikely, bugs of that kind are hard to track down, so we'd
 * like to avoid the possibility.
 * So, on entry to the handler we detect whether we interrupted an
 * existing activation in its critical region -- if so, we pop the current
 * activation and restart the handler using the previous one.
 */
ENTRY(xen_do_hypervisor_callback)   # do_hypervisor_callback(struct *pt_regs)
        CFI_STARTPROC
/*
 * Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
 * see the correct pointer to the pt_regs
 */
        movq %rdi, %rsp            # we don't return, adjust the stack frame
        CFI_ENDPROC
        DEFAULT_FRAME
11:     incl PER_CPU_VAR(irq_count)
        movq %rsp,%rbp
        CFI_DEF_CFA_REGISTER rbp
        cmovzq PER_CPU_VAR(irq_stack_ptr),%rsp
        pushq %rbp                      # backlink for old unwinder
        call xen_evtchn_do_upcall
        popq %rsp
        CFI_DEF_CFA_REGISTER rsp
        decl PER_CPU_VAR(irq_count)
#ifndef CONFIG_PREEMPT
        call xen_maybe_preempt_hcall
#endif
        jmp  error_exit
        CFI_ENDPROC
END(xen_do_hypervisor_callback)

/*
 * Hypervisor uses this for application faults while it executes.
 * We get here for two reasons:
 *  1. Fault while reloading DS, ES, FS or GS
 *  2. Fault while executing IRET
 * Category 1 we do not need to fix up as Xen has already reloaded all segment
 * registers that could be reloaded and zeroed the others.
 * Category 2 we fix up by killing the current process. We cannot use the
 * normal Linux return path in this case because if we use the IRET hypercall
 * to pop the stack frame we end up in an infinite loop of failsafe callbacks.
 * We distinguish between categories by comparing each saved segment register
 * with its current contents: any discrepancy means we in category 1.
 */
ENTRY(xen_failsafe_callback)
        INTR_FRAME 1 (6*8)
        /*CFI_REL_OFFSET gs,GS*/
        /*CFI_REL_OFFSET fs,FS*/
        /*CFI_REL_OFFSET es,ES*/
        /*CFI_REL_OFFSET ds,DS*/
        CFI_REL_OFFSET r11,8
        CFI_REL_OFFSET rcx,0
        movw %ds,%cx
        cmpw %cx,0x10(%rsp)
        CFI_REMEMBER_STATE
        jne 1f
        movw %es,%cx
        cmpw %cx,0x18(%rsp)
        jne 1f
        movw %fs,%cx
        cmpw %cx,0x20(%rsp)
        jne 1f
        movw %gs,%cx
        cmpw %cx,0x28(%rsp)
        jne 1f
        /* All segments match their saved values => Category 2 (Bad IRET). */
        movq (%rsp),%rcx
        CFI_RESTORE rcx
        movq 8(%rsp),%r11
        CFI_RESTORE r11
        addq $0x30,%rsp
        CFI_ADJUST_CFA_OFFSET -0x30
        pushq_cfi $0    /* RIP */
        pushq_cfi %r11
        pushq_cfi %rcx
        jmp general_protection
        CFI_RESTORE_STATE
1:      /* Segment mismatch => Category 1 (Bad segment). Retry the IRET. */
        movq (%rsp),%rcx
        CFI_RESTORE rcx
        movq 8(%rsp),%r11
        CFI_RESTORE r11
        addq $0x30,%rsp
        CFI_ADJUST_CFA_OFFSET -0x30
        pushq_cfi $-1 /* orig_ax = -1 => not a system call */
        ALLOC_PT_GPREGS_ON_STACK
        SAVE_C_REGS
        SAVE_EXTRA_REGS
        jmp error_exit
        CFI_ENDPROC
END(xen_failsafe_callback)

apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
        xen_hvm_callback_vector xen_evtchn_do_upcall

#endif /* CONFIG_XEN */

#if IS_ENABLED(CONFIG_HYPERV)
apicinterrupt3 HYPERVISOR_CALLBACK_VECTOR \
        hyperv_callback_vector hyperv_vector_handler
#endif /* CONFIG_HYPERV */

idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
idtentry int3 do_int3 has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
idtentry stack_segment do_stack_segment has_error_code=1
#ifdef CONFIG_XEN
idtentry xen_debug do_debug has_error_code=0
idtentry xen_int3 do_int3 has_error_code=0
idtentry xen_stack_segment do_stack_segment has_error_code=1
#endif
idtentry general_protection do_general_protection has_error_code=1
trace_idtentry page_fault do_page_fault has_error_code=1
#ifdef CONFIG_KVM_GUEST
idtentry async_page_fault do_async_page_fault has_error_code=1
#endif
#ifdef CONFIG_X86_MCE
idtentry machine_check has_error_code=0 paranoid=1 do_sym=*machine_check_vector(%rip)
#endif

/*
 * Save all registers in pt_regs, and switch gs if needed.
 * Use slow, but surefire "are we in kernel?" check.
 * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
 */
ENTRY(paranoid_entry)
        XCPT_FRAME 1 15*8
        cld
        SAVE_C_REGS 8
        SAVE_EXTRA_REGS 8
        movl $1,%ebx
        movl $MSR_GS_BASE,%ecx
        rdmsr
        testl %edx,%edx
        js 1f   /* negative -> in kernel */
        SWAPGS
        xorl %ebx,%ebx
1:      ret
        CFI_ENDPROC
END(paranoid_entry)

/*
 * "Paranoid" exit path from exception stack.  This is invoked
 * only on return from non-NMI IST interrupts that came
 * from kernel space.
 *
 * We may be returning to very strange contexts (e.g. very early
 * in syscall entry), so checking for preemption here would
 * be complicated.  Fortunately, we there's no good reason
 * to try to handle preemption here.
 */
/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
ENTRY(paranoid_exit)
        DEFAULT_FRAME
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF_DEBUG
        testl %ebx,%ebx                         /* swapgs needed? */
        jnz paranoid_exit_no_swapgs
        TRACE_IRQS_IRETQ
        SWAPGS_UNSAFE_STACK
        jmp paranoid_exit_restore
paranoid_exit_no_swapgs:
        TRACE_IRQS_IRETQ_DEBUG
paranoid_exit_restore:
        RESTORE_EXTRA_REGS
        RESTORE_C_REGS
        REMOVE_PT_GPREGS_FROM_STACK 8
        INTERRUPT_RETURN
        CFI_ENDPROC
END(paranoid_exit)

/*
 * Save all registers in pt_regs, and switch gs if needed.
 * Return: ebx=0: need swapgs on exit, ebx=1: otherwise
 */
ENTRY(error_entry)
        XCPT_FRAME 1 15*8
        cld
        SAVE_C_REGS 8
        SAVE_EXTRA_REGS 8
        xorl %ebx,%ebx
        testl $3,CS+8(%rsp)
        je error_kernelspace
error_swapgs:
        SWAPGS
error_sti:
        TRACE_IRQS_OFF
        ret

        /*
         * There are two places in the kernel that can potentially fault with
         * usergs. Handle them here.  B stepping K8s sometimes report a
         * truncated RIP for IRET exceptions returning to compat mode. Check
         * for these here too.
         */
error_kernelspace:
        CFI_REL_OFFSET rcx, RCX+8
        incl %ebx
        leaq native_irq_return_iret(%rip),%rcx
        cmpq %rcx,RIP+8(%rsp)
        je error_bad_iret
        movl %ecx,%eax  /* zero extend */
        cmpq %rax,RIP+8(%rsp)
        je bstep_iret
        cmpq $gs_change,RIP+8(%rsp)
        je error_swapgs
        jmp error_sti

bstep_iret:
        /* Fix truncated RIP */
        movq %rcx,RIP+8(%rsp)
        /* fall through */

error_bad_iret:
        SWAPGS
        mov %rsp,%rdi
        call fixup_bad_iret
        mov %rax,%rsp
        decl %ebx       /* Return to usergs */
        jmp error_sti
        CFI_ENDPROC
END(error_entry)


/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */
ENTRY(error_exit)
        DEFAULT_FRAME
        movl %ebx,%eax
        RESTORE_EXTRA_REGS
        DISABLE_INTERRUPTS(CLBR_NONE)
        TRACE_IRQS_OFF
        GET_THREAD_INFO(%rcx)
        testl %eax,%eax
        jne retint_kernel
        LOCKDEP_SYS_EXIT_IRQ
        movl TI_flags(%rcx),%edx
        movl $_TIF_WORK_MASK,%edi
        andl %edi,%edx
        jnz retint_careful
        jmp retint_swapgs
        CFI_ENDPROC
END(error_exit)

/* Runs on exception stack */
ENTRY(nmi)
        INTR_FRAME
        PARAVIRT_ADJUST_EXCEPTION_FRAME
        /*
         * We allow breakpoints in NMIs. If a breakpoint occurs, then
         * the iretq it performs will take us out of NMI context.
         * This means that we can have nested NMIs where the next
         * NMI is using the top of the stack of the previous NMI. We
         * can't let it execute because the nested NMI will corrupt the
         * stack of the previous NMI. NMI handlers are not re-entrant
         * anyway.
         *
         * To handle this case we do the following:
         *  Check the a special location on the stack that contains
         *  a variable that is set when NMIs are executing.
         *  The interrupted task's stack is also checked to see if it
         *  is an NMI stack.
         *  If the variable is not set and the stack is not the NMI
         *  stack then:
         *    o Set the special variable on the stack
         *    o Copy the interrupt frame into a "saved" location on the stack
         *    o Copy the interrupt frame into a "copy" location on the stack
         *    o Continue processing the NMI
         *  If the variable is set or the previous stack is the NMI stack:
         *    o Modify the "copy" location to jump to the repeate_nmi
         *    o return back to the first NMI
         *
         * Now on exit of the first NMI, we first clear the stack variable
         * The NMI stack will tell any nested NMIs at that point that it is
         * nested. Then we pop the stack normally with iret, and if there was
         * a nested NMI that updated the copy interrupt stack frame, a
         * jump will be made to the repeat_nmi code that will handle the second
         * NMI.
         */

        /* Use %rdx as our temp variable throughout */
        pushq_cfi %rdx
        CFI_REL_OFFSET rdx, 0

        /*
         * If %cs was not the kernel segment, then the NMI triggered in user
         * space, which means it is definitely not nested.
         */
        cmpl $__KERNEL_CS, 16(%rsp)
        jne first_nmi

        /*
         * Check the special variable on the stack to see if NMIs are
         * executing.
         */
        cmpl $1, -8(%rsp)
        je nested_nmi

        /*
         * Now test if the previous stack was an NMI stack.
         * We need the double check. We check the NMI stack to satisfy the
         * race when the first NMI clears the variable before returning.
         * We check the variable because the first NMI could be in a
         * breakpoint routine using a breakpoint stack.
         */
        lea     6*8(%rsp), %rdx
        /* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
        cmpq    %rdx, 4*8(%rsp)
        /* If the stack pointer is above the NMI stack, this is a normal NMI */
        ja      first_nmi
        subq    $EXCEPTION_STKSZ, %rdx
        cmpq    %rdx, 4*8(%rsp)
        /* If it is below the NMI stack, it is a normal NMI */
        jb      first_nmi
        /* Ah, it is within the NMI stack, treat it as nested */

        CFI_REMEMBER_STATE

nested_nmi:
        /*
         * Do nothing if we interrupted the fixup in repeat_nmi.
         * It's about to repeat the NMI handler, so we are fine
         * with ignoring this one.
         */
        movq $repeat_nmi, %rdx
        cmpq 8(%rsp), %rdx
        ja 1f
        movq $end_repeat_nmi, %rdx
        cmpq 8(%rsp), %rdx
        ja nested_nmi_out

1:
        /* Set up the interrupted NMIs stack to jump to repeat_nmi */
        leaq -1*8(%rsp), %rdx
        movq %rdx, %rsp
        CFI_ADJUST_CFA_OFFSET 1*8
        leaq -10*8(%rsp), %rdx
        pushq_cfi $__KERNEL_DS
        pushq_cfi %rdx
        pushfq_cfi
        pushq_cfi $__KERNEL_CS
        pushq_cfi $repeat_nmi

        /* Put stack back */
        addq $(6*8), %rsp
        CFI_ADJUST_CFA_OFFSET -6*8

nested_nmi_out:
        popq_cfi %rdx
        CFI_RESTORE rdx

        /* No need to check faults here */
        INTERRUPT_RETURN

        CFI_RESTORE_STATE
first_nmi:
        /*
         * Because nested NMIs will use the pushed location that we
         * stored in rdx, we must keep that space available.
         * Here's what our stack frame will look like:
         * +-------------------------+
         * | original SS             |
         * | original Return RSP     |
         * | original RFLAGS         |
         * | original CS             |
         * | original RIP            |
         * +-------------------------+
         * | temp storage for rdx    |
         * +-------------------------+
         * | NMI executing variable  |
         * +-------------------------+
         * | copied SS               |
         * | copied Return RSP       |
         * | copied RFLAGS           |
         * | copied CS               |
         * | copied RIP              |
         * +-------------------------+
         * | Saved SS                |
         * | Saved Return RSP        |
         * | Saved RFLAGS            |
         * | Saved CS                |
         * | Saved RIP               |
         * +-------------------------+
         * | pt_regs                 |
         * +-------------------------+
         *
         * The saved stack frame is used to fix up the copied stack frame
         * that a nested NMI may change to make the interrupted NMI iret jump
         * to the repeat_nmi. The original stack frame and the temp storage
         * is also used by nested NMIs and can not be trusted on exit.
         */
        /* Do not pop rdx, nested NMIs will corrupt that part of the stack */
        movq (%rsp), %rdx
        CFI_RESTORE rdx

        /* Set the NMI executing variable on the stack. */
        pushq_cfi $1

        /*
         * Leave room for the "copied" frame
         */
        subq $(5*8), %rsp
        CFI_ADJUST_CFA_OFFSET 5*8

        /* Copy the stack frame to the Saved frame */
        .rept 5
        pushq_cfi 11*8(%rsp)
        .endr
        CFI_DEF_CFA_OFFSET 5*8

        /* Everything up to here is safe from nested NMIs */

        /*
         * If there was a nested NMI, the first NMI's iret will return
         * here. But NMIs are still enabled and we can take another
         * nested NMI. The nested NMI checks the interrupted RIP to see
         * if it is between repeat_nmi and end_repeat_nmi, and if so
         * it will just return, as we are about to repeat an NMI anyway.
         * This makes it safe to copy to the stack frame that a nested
         * NMI will update.
         */
repeat_nmi:
        /*
         * Update the stack variable to say we are still in NMI (the update
         * is benign for the non-repeat case, where 1 was pushed just above
         * to this very stack slot).
         */
        movq $1, 10*8(%rsp)

        /* Make another copy, this one may be modified by nested NMIs */
        addq $(10*8), %rsp
        CFI_ADJUST_CFA_OFFSET -10*8
        .rept 5
        pushq_cfi -6*8(%rsp)
        .endr
        subq $(5*8), %rsp
        CFI_DEF_CFA_OFFSET 5*8
end_repeat_nmi:

        /*
         * Everything below this point can be preempted by a nested
         * NMI if the first NMI took an exception and reset our iret stack
         * so that we repeat another NMI.
         */
        pushq_cfi $-1           /* ORIG_RAX: no syscall to restart */
        ALLOC_PT_GPREGS_ON_STACK

        /*
         * Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
         * as we should not be calling schedule in NMI context.
         * Even with normal interrupts enabled. An NMI should not be
         * setting NEED_RESCHED or anything that normal interrupts and
         * exceptions might do.
         */
        call paranoid_entry
        DEFAULT_FRAME 0

        /*
         * Save off the CR2 register. If we take a page fault in the NMI then
         * it could corrupt the CR2 value. If the NMI preempts a page fault
         * handler before it was able to read the CR2 register, and then the
         * NMI itself takes a page fault, the page fault that was preempted
         * will read the information from the NMI page fault and not the
         * origin fault. Save it off and restore it if it changes.
         * Use the r12 callee-saved register.
         */
        movq %cr2, %r12

        /* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
        movq %rsp,%rdi
        movq $-1,%rsi
        call do_nmi

        /* Did the NMI take a page fault? Restore cr2 if it did */
        movq %cr2, %rcx
        cmpq %rcx, %r12
        je 1f
        movq %r12, %cr2
1:
        
        testl %ebx,%ebx                         /* swapgs needed? */
        jnz nmi_restore
nmi_swapgs:
        SWAPGS_UNSAFE_STACK
nmi_restore:
        RESTORE_EXTRA_REGS
        RESTORE_C_REGS
        /* Pop the extra iret frame at once */
        REMOVE_PT_GPREGS_FROM_STACK 6*8

        /* Clear the NMI executing stack variable */
        movq $0, 5*8(%rsp)
        jmp irq_return
        CFI_ENDPROC
END(nmi)

ENTRY(ignore_sysret)
        CFI_STARTPROC
        mov $-ENOSYS,%eax
        sysret
        CFI_ENDPROC
END(ignore_sysret)