From 0e0a53c551317654e2d7885fdfd23299fee99b6b Mon Sep 17 00:00:00 2001 From: Paolo Bonzini <pbonzini@redhat.com> Date: Wed, 22 Aug 2018 19:59:33 +0200 Subject: [PATCH] kvm: x86: optimize dr6 restore The quote from the comment almost says it all: we are currently zeroing the guest dr6 in kvm_arch_vcpu_put, because do_debug expects it. However, the host %dr6 is either: - zero because the guest hasn't run after kvm_arch_vcpu_load - written from vcpu->arch.dr6 by vcpu_enter_guest - written by the guest and copied to vcpu->arch.dr6 by ->sync_dirty_debug_regs(). Therefore, we can skip the write if vcpu->arch.dr6 is already zero. We may do extra useless writes if vcpu->arch.dr6 is nonzero but the guest hasn't run; however that is less important for performance. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- arch/x86/kvm/x86.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 20a667da0a31a..92159db9cf719 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3186,11 +3186,16 @@ 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. + * 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. */ - set_debugreg(0, 6); + if (vcpu->arch.dr6) + set_debugreg(0, 6); } static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, -- GitLab