Re: [PATCH v4 4/4] KVM: SVM: Add Bus Lock Detect support

[Ravi Bangoria]

>> @@ -3158,6 +3159,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>  		if (data & DEBUGCTL_RESERVED_BITS)
> 
> Not your code, but why does DEBUGCTL_RESERVED_BITS = ~0x3f!?!?  That means the
> introduction of the below check, which is architecturally correct, has the
> potential to break guests.  *sigh*
> 
> I doubt it will cause a problem, but it's something to look out for.
This dates back to 2008: https://git.kernel.org/torvalds/c/24e09cbf480a7

The legacy definition[1] of DEBUGCTL MSR is:

  5:2   PB: performance monitor pin control. Read-write. Reset: 0h.
        This field does not control any hardware.
  1     BTF. Read-write. Reset: 0. 1=Enable branch single step.
  0     LBR. Read-write. Reset: 0. 1=Enable last branch record.

[1]: https://bugzilla.kernel.org/attachment.cgi?id=287389

Thanks,
Ravi