Re: [PATCH v8 1/7] target/i386/kvm: set KVM_PMU_CAP_DISABLE if "-pmu" is configured

[Sandipan Das <sandipan.das@amd.com>]

On 01-10-2026 16:42, Sandipan Das wrote:
> On 01-10-2026 16:35, Michael Tokarev wrote:
>> On 10/1/26 13:43, Sandipan Das wrote:
>>> On 01-10-2026 15:30, Michael Tokarev wrote:
>>>> On 10/1/26 11:54, Sandipan Das wrote:
>>>>> On 01-10-2026 14:02, Michael Tokarev wrote:
>>>> ...
>>>>
>>>>>> When booting windows11 VM with -cpu EPYC-Rome-v4 -enable-kvm, the
>>>>>> guest enters tight CPU loop while displaying a boot screen, before
>>>>>> it starts showing up the rotating circle (which shows boot progress).
>>>>
>>>>
>>>>> What happens if you use "-cpu EPYC-Rome-v4,+pmu"?
>>>>
>>>> With that, it boots fine, just like before this patch.
>>>> I checked current master too - same, +pmu fixes it.
>>>>
>>> A guest OS cannot expect a functioning PMU if the hypervisor has disabled
>>> it. Linux runs a basic sanity test to determine this. See:
>>> https://github.com/torvalds/linux/blob/master/arch/x86/events/core.c#L268
>>
>> So.. what do we do with all this?  :)
>>
>> I kinda see the intention of the original change, but it results in
>> non-bootable systems (linux too, as I think you demonstrate above).
>>
> 
> Because of the sanity test, Linux will boot but not register the "cpu" PMU.
> Perhaps others should also run a similar sanity test on PMU hardware rather
> than making assumptions and failing to boot.

After looking at kvm_msr and kvm_cpuid traces, I see that Windows is indeed
getting #GPs upon accessing MSRs 0xc0010200 and 0xc0010201 but it is not at
fault here since it expects these MSRs to be available as guest CPUID
0x80000001[ECX].PerfCtrExtCore is set. This feature bit should have been
cleared by QEMU when "pmu=off".

It boots up when "pmu=off" is combined with "perfctr-core=off" and
"perfmon-v2=off". The missing pieces here are the following:

https://lore.kernel.org/qemu-devel/20251111061532.36702-2-dongli.zhang@oracle.com/
https://lore.kernel.org/qemu-devel/20251111061532.36702-3-dongli.zhang@oracle.com/

Dongli, Zhao,

I see a discussion about compat array at
https://lore.kernel.org/qemu-devel/aUEXkDDOba+oZ4v+@intel.com/

Is this ready for the two patches above to be pulled in?