Re: [PATCH 2/5] KVM: selftests: Only validate counts for hardware-supported arch events

[Paolo Bonzini]

On 1/18/25 01:39, Sean Christopherson wrote:
> On Sat, Jan 18, 2025, Mingwei Zhang wrote:
> > On Fri, Jan 17, 2025, Sean Christopherson wrote:
> > > @@ -582,18 +585,26 @@ static void test_intel_counters(void)
> > >   
> > >   	/*
> > >   	 * Detect the existence of events that aren't supported by selftests.
> > > -	 * This will (obviously) fail any time the kernel adds support for a
> > > -	 * new event, but it's worth paying that price to keep the test fresh.
> > > +	 * This will (obviously) fail any time hardware adds support for a new
> > > +	 * event, but it's worth paying that price to keep the test fresh.
> > >   	 */
> > >   	TEST_ASSERT(nr_arch_events <= NR_INTEL_ARCH_EVENTS,
> > >   		    "New architectural event(s) detected; please update this test (length = %u, mask = %x)",
> > > -		    nr_arch_events, kvm_cpu_property(X86_PROPERTY_PMU_EVENTS_MASK));
> > > +		    nr_arch_events, this_cpu_property(X86_PROPERTY_PMU_EVENTS_MASK));
> >
> > This is where it would make troubles for us (all companies that might be
> > using the selftest in upstream kernel and having a new hardware). In
> > this case when we get new hardware, the test will fail in the downstream
> > kernel. We will have to wait until the fix is ready, and backport it
> > downstream, re-test it.... It takes lots of extra work.
>
> If Intel can't upstream what should be a *very* simple patch to enumerate the
> new encoding and its expected count in advance of hardware being shipped to
> partners, then we have bigger problems.

Conceptually I have bigger problems with people running stable kernels 
than people running on really really new hardware.

However the intersection of running a pretty old kernel on a very new 
bare metal x86 system is relatively small but nonzero (those pesky 
Debian users); it may happen with cloud instances but then the 
intersection of running old selftests in a nested virt environment is 
probably even smaller.

I am not too happy about the assertion, but it does seem like the lesser 
evil.

Paolo