Re: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as a perf event clock

[Adrian Hunter <adrian.hunter@intel.com>]

On 7.3.2022 16.42, Peter Zijlstra wrote:
> On Mon, Mar 07, 2022 at 02:36:03PM +0200, Adrian Hunter wrote:
> 
>>> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
>>> index 4420499f7bb4..a1f179ed39bf 100644
>>> --- a/arch/x86/kernel/paravirt.c
>>> +++ b/arch/x86/kernel/paravirt.c
>>> @@ -145,6 +145,15 @@ DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
>>>  
>>>  void paravirt_set_sched_clock(u64 (*func)(void))
>>>  {
>>> +	/*
>>> +	 * Anything with ART on promises to have sane TSC, otherwise the whole
>>> +	 * ART thing is useless. In order to make ART useful for guests, we
>>> +	 * should continue to use the TSC. As such, ignore any paravirt
>>> +	 * muckery.
>>> +	 */
>>> +	if (cpu_feature_enabled(X86_FEATURE_ART))
>>
>> Does not seem to work because the feature X86_FEATURE_ART does not seem to get set.
>> Possibly because detect_art() excludes anything running on a hypervisor.
> 
> Simple enough to delete that clause I suppose. Christopher, what is
> needed to make that go away? I suppose the guest needs to be aware of
> the active TSC scaling parameters to make it work ?

There is also not X86_FEATURE_NONSTOP_TSC nor values for art_to_tsc_denominator
or art_to_tsc_numerator.  Also, from the VM's point of view, TSC will jump
forwards every VM-Exit / VM-Entry unless the hypervisor changes the offset
every VM-Entry, which KVM does not, so it still cannot be used as a stable
clocksource.