Re: [PATCH v4] perf/x86/amd: Don't touch the Host-only bit inside the guest hypervisor

Peter Zijlstra
2022-04-01T13:07:04Z

On Fri, Apr 01, 2022 at 04:29:11PM +0800, Dongli Si wrote:
> On 28/03/2022 14:03, Peter Zijlstra wrote:
> > Better I suppose, but I think the comments can be improved by covering
> > the 'why' of things. We can all read the code to see the what of it.
>
> I will add comments to the code to explain 'why'.
>
> > Anyway, doesn't this also affect behaviour? I'm guessing this HO bit is
> > only set by perf-record for events it wants to record on the host. But
> > by not setting it, we'll also record the activity of the guest.
>
> I think the HO/GO bit can only be set on the host, and should only be set
> if SVM is enabled.
>
> When the SVM is disabled, set the HO/GO bit will cause the performance
> counters to not work.
>
> Set the HO/GO bit inside the guest will cause the guest emitted
> "unchecked MSR access error" warning, can be triggered by running
> "perf stat -e instructions:G ls" in the guest, because this will set
> the GO bit in the guest, and perf_ctr_virt_mask just mask the HO bit.
>
> My patch does not affect the host, it just fixes the bug in the guest.
>
> > So suppose we create a CPU wide HO event, then it will only count L0
> > activity, right? Any L1 (or higher) activite will be invisible.
>
> I don't quite understand your question.
>
> > But with this change on, the L1 HV doesn't provide these same semantics,
> > it's guest will be included in that host counter.
>
> I don't think applying this patch will cause L2 guests to be included in
> the host counter.
>
> > Or is there additional counter {dis,en}abling on virt enter,exit (resp.)
> > to achieve these semantics?
>
> I don't think there is such a counter.

If SVM enter/exit don't twiddle with counter EN bits, how is all this
supposed to work consistently then?