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([2001:b07:6468:f312:63a7:c72e:ea0e:6045]) by smtp.gmail.com with ESMTPSA id o23sm9346857eds.75.2021.08.10.02.43.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Aug 2021 02:43:01 -0700 (PDT) Subject: Re: [PATCH V2 3/3] KVM: X86: Reset DR6 only when KVM_DEBUGREG_WONT_EXIT To: Lai Jiangshan , linux-kernel@vger.kernel.org Cc: Lai Jiangshan , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org References: <20210809174307.145263-1-jiangshanlai@gmail.com> <20210809174307.145263-3-jiangshanlai@gmail.com> From: Paolo Bonzini Message-ID: <0a5243de-9fc2-388f-ea4d-377261d64430@redhat.com> Date: Tue, 10 Aug 2021 11:42:59 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210809174307.145263-3-jiangshanlai@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/08/21 19:43, Lai Jiangshan wrote: > From: Lai Jiangshan > > 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 > --- > 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); > } > > /* >