Re: [PATCH v2 3/5] KVM: SVM: Manually context switch DEBUGCTL if LBR virtualization is disabled

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 27, 2025, Ravi Bangoria wrote:
> Hi Sean,
> 
> > @@ -4265,6 +4265,16 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
> >  	clgi();
> >  	kvm_load_guest_xsave_state(vcpu);
> >  
> > +	/*
> > +	 * Hardware only context switches DEBUGCTL if LBR virtualization is
> > +	 * enabled.  Manually load DEBUGCTL if necessary (and restore it after
> > +	 * VM-Exit), as running with the host's DEBUGCTL can negatively affect
> > +	 * guest state and can even be fatal, e.g. due to Bus Lock Detect.
> > +	 */
> > +	if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK) &&
> > +	    vcpu->arch.host_debugctl != svm->vmcb->save.dbgctl)
> > +		update_debugctlmsr(0);
> 
>                 ^^^^^^^^^^^^^^^^^^^^^
> You mean:
>                 update_debugctlmsr(svm->vmcb->save.dbgctl);
> ?

Argh, yes.

> Somewhat related but independent: CPU automatically clears DEBUGCTL[BTF]
> on #DB exception. So, when DEBUGCTL is save/restored by KVM (i.e. when
> LBR virtualization is disabled), it's KVM's responsibility to clear
> DEBUGCTL[BTF].
> ---
> @@ -2090,6 +2090,14 @@ static int db_interception(struct kvm_vcpu *vcpu)
>  	      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
>  		!svm->nmi_singlestep) {
>  		u32 payload = svm->vmcb->save.dr6 ^ DR6_ACTIVE_LOW;
> +
> +		/*
> +		 * CPU automatically clears DEBUGCTL[BTF] on #DB exception.
> +		 * Simulate it when DEBUGCTL isn't auto save/restored.
> +		 */
> +		if (!(svm->vmcb->control.virt_ext & LBR_CTL_ENABLE_MASK))
> +			svm->vmcb->save.dbgctl &= ~0x2;

Any reason not to clear is unconditionally?

		svm->vmcb->save.dbgctl &= ~DEBUGCTLMSR_BTF;

>  		kvm_queue_exception_p(vcpu, DB_VECTOR, payload);
>  		return 1;
>  	}
> ---
> 
> Thanks,
> Ravi