>> 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;
No particular reason, just that HW would have already done it when LBRV
is enabled.
Thanks,
Ravi
