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

[Paolo Bonzini]

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