Revert "kvm: x86: optimize dr6 restore"

This reverts commit 0e0a53c551317654e2d7885fdfd23299fee99b6b.

As Christian Ehrhardt noted:

  The most common case is that vcpu->arch.dr6 and the host's %dr6 value
  are not related at all because ->switch_db_regs is zero. To do this
  all correctly, we must handle the case where the guest leaves an arbitrary
  unused value in vcpu->arch.dr6 before disabling breakpoints again.

  However, this means that vcpu->arch.dr6 is not suitable to detect the
  need for a %dr6 clear.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index bdcb5babfb68ca4705fdda7a7d568d63f94cfcdb..66d66d77caee5c7b761ae4eda2f0caf4643987e1 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3271,16 +3271,11 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
        kvm_x86_ops->vcpu_put(vcpu);
        vcpu->arch.last_host_tsc = rdtsc();
        /*
-        * Here dr6 is either zero or, if the guest has run and userspace
-        * has not set any breakpoints or watchpoints, it can be set to
-        * the guest dr6 (stored in vcpu->arch.dr6). do_debug expects dr6
-        * to be cleared after it runs, so clear the host register.  However,
-        * MOV to DR can be expensive when running nested, omit it if
-        * vcpu->arch.dr6 is already zero: in that case, the host dr6 cannot
-        * currently be nonzero.
+        * If userspace has set any breakpoints or watchpoints, dr6 is restored
+        * on every vmexit, but if not, we might have a stale dr6 from the
+        * guest. do_debug expects dr6 to be cleared after it runs, do the same.
         */
-       if (vcpu->arch.dr6)
-               set_debugreg(0, 6);
+       set_debugreg(0, 6);
 }
 
 static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,