Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT

Paolo Bonzini
2021-08-10T10:35:50Z

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
>