Re: [PATCH v5 09/44] perf/x86: Switch LVTPC to/from mediated PMI vector on guest load/put context

[Peter Zijlstra]

On Mon, Aug 18, 2025 at 08:25:34AM -0700, Sean Christopherson wrote:

> > OK, so *IF* doing the VM-exit during PMI is sound, this is something
> > that needs a comment somewhere. 
> 
> I'm a bit lost here.  Are you essentially asking if it's ok to take a VM-Exit
> while the guest is handling a PMI?  If so, that _has_ to work, because there are
> myriad things that can/will trigger a VM-Exit at any point while the guest is
> active.

Yes, that's what I'm asking. Why is this VM-exit during PMI nonsense not
subject to the same failures that mandates the mid/late PMI ACK.

And yes, I realize this needs to work. But so far I'm not sure I
understand why that is a safe thing to do.

Like I wrote, I suspect writing all the PMU MSRs serializes things
sufficiently, but if that is the case, that needs to be explicitly
mentioned. Because that also doesn't explain why we needs mid-ack
instead of late-ack on ADL e-cores for instance.

Could it perhaps be that we don't let the guests do PEBS because DS
doesn't virtualize? And thus we don't have the malformed PEBS record?