Re: [PATCH v2 7/7] KVM: VMX: Simplify capability check when handling PERF_CAPABILITIES write

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 04, 2022, Like Xu wrote:
> On 4/8/2022 3:26 am, Sean Christopherson wrote:
> > Explicitly check for the absence of host support for LBRs or PEBS when
> > userspace attempts to enable said features by writing PERF_CAPABILITIES.
> > Comparing host support against the incoming value is unnecessary and
> > weird since the checks are buried inside an if-statement that verifies
> > userspace wants to enable the feature.
> 
> If you mean this part in the KVM:
> 
> 	case MSR_IA32_PERF_CAPABILITIES: {
> 		...
> 		if (data & ~msr_ent.data)
> 			return 1;
> 		...
> 
> then this patch brings a flaw, for example:
> 
> a user space can successfully set 0x1 on a host that reports a value of 0x5,
> which should not happen since the semantics of 0x1 and 0x5 for LBR_FMT
> may be completely different from the guest LBR driver's perspective.

/facepalm

I keep forgetting the caps need to match the host exactly.  Thanks!