Change efdab992: "KVM: x86: fix escape of guest dr6 to the host" has negative effects if Linux runs as the L1 guest in a nested VM setup: Each call to kvm_arch_vcpu_put causes an additional L1->L0 exit due to the %dr6 write even if neither L1 nor L2 uses debug registers. As the current hardware value of %dr6 is already in vcpu->arch.dr6 we can avoid the debug register write if all non-reserved bits have their default value. Update the comment and reformat it while there. Signed-off-by: Christian Ehrhardt <lk@xxxxxxx> --- arch/x86/kvm/x86.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index ca717737347e..fcca60adc037 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3185,11 +3185,13 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) kvm_x86_ops->vcpu_put(vcpu); vcpu->arch.last_host_tsc = rdtsc(); /* - * 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 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. Thus reset the hardware value of dr6 to + * its reset value. Avoid the debug register write if possible. */ - set_debugreg(0, 6); + if (vcpu->arch.dr6 != DR6_INIT) + set_debugreg(DR6_INIT, 6); } static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, -- 2.17.1
