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([2001:b07:6468:f312:63a7:c72e:ea0e:6045]) by smtp.gmail.com with ESMTPSA id q11sm6710763ejb.10.2021.08.10.03.35.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 10 Aug 2021 03:35:43 -0700 (PDT) Subject: Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT To: Lai Jiangshan , Lai Jiangshan , linux-kernel@vger.kernel.org Cc: 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> <20210809174307.145263-2-jiangshanlai@gmail.com> <68ed0f5c-40f1-c240-4ad1-b435568cf753@redhat.com> <45fef019-8bd9-2acb-bd53-1243a8a07c4e@linux.alibaba.com> From: Paolo Bonzini Message-ID: Date: Tue, 10 Aug 2021 12:35:42 +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: <45fef019-8bd9-2acb-bd53-1243a8a07c4e@linux.alibaba.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/08/21 12:30, Lai Jiangshan wrote: >> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c >> index ae8e62df16dd..21a3ef3012cf 100644 >> --- a/arch/x86/kvm/vmx/vmx.c >> +++ b/arch/x86/kvm/vmx/vmx.c >> @@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu >> *vcpu) >>           vmx->loaded_vmcs->host_state.cr4 = cr4; >>       } >> >> +    /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */ >> +    if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT) >> +        set_debugreg(vcpu->arch.dr6, 6); > > > I also noticed the related code in svm.c, but I refrained myself > to add a new branch in vmx_vcpu_run().  But after I see you put > the code of resetting dr6 in vmx_sync_dirty_debug_regs(), the whole > solution is much clean and better. > > And if any chance you are also concern about the additional branch, > could you add a new callback to set dr6 and call the callback from > x86.c when KVM_DEBUGREG_WONT_EXIT. The extra branch should be well predicted, and the idea you sketched below would cause DR6 to be marked uselessly as dirty in SVM, so I think this is cleaner. Let's add an "unlikely" around it too. Paolo > The possible implementation of the callback: > for vmx: set_debugreg(vcpu->arch.dr6, 6); > for svm: svm_set_dr6(svm, vcpu->arch.dr6); >          and always do svm_set_dr6(svm, DR6_ACTIVE_LOW); at the end of the >          svm_handle_exit(). > > Thanks > Lai > >> + >>       /* When single-stepping over STI and MOV SS, we must clear the >>        * corresponding interruptibility bits in the guest state. >> Otherwise >>        * vmentry fails as it then expects bit 14 (BS) in pending debug >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index a111899ab2b4..fbc536b21585 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) >>           set_debugreg(vcpu->arch.eff_db[1], 1); >>           set_debugreg(vcpu->arch.eff_db[2], 2); >>           set_debugreg(vcpu->arch.eff_db[3], 3); >> -        set_debugreg(vcpu->arch.dr6, 6); >>       } else if (unlikely(hw_breakpoint_active())) { >>           set_debugreg(0, 7); >>       } >> >> Paolo >