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Date: Tue, 10 Aug 2021 11:42:59 +0200 From: Paolo Bonzini <pbonzini@...hat.com> To: Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org Cc: Lai Jiangshan <laijs@...ux.alibaba.com>, Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org Subject: Re: [PATCH V2 3/3] KVM: X86: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT On 09/08/21 19:43, Lai Jiangshan wrote: > From: Lai Jiangshan <laijs@...ux.alibaba.com> > > The commit efdab992813fb ("KVM: x86: fix escape of guest dr6 to the host") > fixed a bug by reseting DR6 unconditionally when the vcpu being scheduled out. > > But writing to debug registers is slow, and it can be shown in perf results > sometimes even neither the host nor the guest activate breakpoints. > > It'd be better to reset it conditionally and this patch moves the code of > reseting DR6 to the path of VM-exit and only reset it when > KVM_DEBUGREG_WONT_EXIT which is the only case that DR6 is guest value. > > Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com> > --- > arch/x86/kvm/x86.c | 8 ++------ > 1 file changed, 2 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index d2aa49722064..f40cdd7687d8 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -4309,12 +4309,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) > > static_call(kvm_x86_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. > - */ > - set_debugreg(0, 6); > } > > static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu, > @@ -9630,6 +9624,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > static_call(kvm_x86_sync_dirty_debug_regs)(vcpu); > kvm_update_dr0123(vcpu); > kvm_update_dr7(vcpu); > + /* Reset Dr6 which is guest value. */ Better keep the rationale from the original comment, /* * do_debug expects dr6 to be cleared after it runs, so do * not leave the guest value in the host DR6. */ Paolo > + set_debugreg(DR6_RESERVED, 6); > } > > /* >
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