arm64: entry: prepare ret_to_user for function call

In a subsequent patch ret_to_user will need to make a C function call
(in some configurations) which may clobber x0-x18 at the start of the
finish_ret_to_user block, before enable_step_tsk consumes the flags
loaded into x1.

In preparation for this, let's load the flags into x19, which is
preserved across C function calls. This avoids a redundant reload of the
flags and ensures we operate on a consistent shapshot regardless.

There should be no functional change as a result of this patch. At this
point of the entry/exit paths we only need to preserve x28 (tsk) and the
sp, and x19 is free for this use.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201130115950.22492-6-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index b295fb912b12b2993e1723a588874a2a6a092b19..84aec600eeed4ccee569f379ee0d4f7ab8bb0e84 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -774,13 +774,13 @@ SYM_CODE_END(el0_error)
 SYM_CODE_START_LOCAL(ret_to_user)
        disable_daif
        gic_prio_kentry_setup tmp=x3
-       ldr     x1, [tsk, #TSK_TI_FLAGS]
-       and     x2, x1, #_TIF_WORK_MASK
+       ldr     x19, [tsk, #TSK_TI_FLAGS]
+       and     x2, x19, #_TIF_WORK_MASK
        cbnz    x2, work_pending
 finish_ret_to_user:
        /* Ignore asynchronous tag check faults in the uaccess routines */
        clear_mte_async_tcf
-       enable_step_tsk x1, x2
+       enable_step_tsk x19, x2
 #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
        bl      stackleak_erase
 #endif
@@ -791,11 +791,12 @@ finish_ret_to_user:
  */
 work_pending:
        mov     x0, sp                          // 'regs'
+       mov     x1, x19
        bl      do_notify_resume
 #ifdef CONFIG_TRACE_IRQFLAGS
        bl      trace_hardirqs_on               // enabled while in userspace
 #endif
-       ldr     x1, [tsk, #TSK_TI_FLAGS]        // re-check for single-step
+       ldr     x19, [tsk, #TSK_TI_FLAGS]       // re-check for single-step
        b       finish_ret_to_user
 SYM_CODE_END(ret_to_user)