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Revert "kvm: x86: optimize dr6 restore" 

This reverts commit 0e0a53c551.

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>
This commit is contained in:
Radim Krčmář 2018-10-23 16:31:38 +02:00
parent 574c0cfbc7
commit f9dcf08e20
1 changed files with 4 additions and 9 deletions
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11
arch/x86/kvm/x86.c
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@ -3271,15 +3271,10 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
kvm_x86_ops->vcpu_put(vcpu); kvm_x86_ops->vcpu_put(vcpu);
vcpu->arch.last_host_tsc = rdtsc(); vcpu->arch.last_host_tsc = rdtsc();
/* /*
* Here dr6 is either zero or, if the guest has run and userspace * If userspace has set any breakpoints or watchpoints, dr6 is restored
* has not set any breakpoints or watchpoints, it can be set to * on every vmexit, but if not, we might have a stale dr6 from the
* the guest dr6 (stored in vcpu->arch.dr6). do_debug expects dr6 * guest. do_debug expects dr6 to be cleared after it runs, do the same.
* 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 (vcpu->arch.dr6)
set_debugreg(0, 6); set_debugreg(0, 6);
} }