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
