Re: [kvm-unit-tests Patch v2 4/5] x86: pmu: Support validation for Intel PMU fixed counter 3

[Jim Mattson]

On Tue, Oct 31, 2023 at 7:33 PM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
>
>
> On 11/1/2023 2:47 AM, Jim Mattson wrote:
> > On Tue, Oct 31, 2023 at 2:22 AM Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
> >> Intel CPUs, like Sapphire Rapids, introduces a new fixed counter
> >> (fixed counter 3) to counter/sample topdown.slots event, but current
> >> code still doesn't cover this new fixed counter.
> >>
> >> So this patch adds code to validate this new fixed counter can count
> >> slots event correctly.
> > I'm not convinced that this actually validates anything.
> >
> > Suppose, for example, that KVM used fixed counter 1 when the guest
> > asked for fixed counter 3. Wouldn't this test still pass?
>
>
> Per my understanding, as long as the KVM returns a valid count in the
> reasonable count range, we can think KVM works correctly. We don't need
> to entangle on how KVM really uses the HW, it could be impossible and
> unnecessary.

Now, I see how the Pentium FDIV bug escaped notice. Hey, the numbers
are in a reasonable range. What's everyone upset about?

> Yeah, currently the predefined valid count range may be some kind of
> loose since I want to cover as much as hardwares and avoid to cause
> regression. Especially after introducing the random jump and clflush
> instructions, the cycles and slots become much more hard to predict.
> Maybe we can have a comparable restricted count range in the initial
> change, and we can loosen the restriction then if we encounter a failure
> on some specific hardware. do you think it's better? Thanks.

I think the test is essentially useless, and should probably just be
deleted, so that it doesn't give a false sense of confidence.