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 16:35, 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.