linux-kernel - Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT

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Message-ID: <f5967e16-3910-5604-7890-9a1741045ce8@redhat.com>
Date:   Tue, 10 Aug 2021 12:35:42 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     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 2/3] KVM: X86: Set the hardware DR6 only when
 KVM_DEBUGREG_WONT_EXIT

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
>