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Message-ID: <013b5425-2a60-e4d4-b846-444a576f2b28@intel.com>
Date:   Tue, 8 Mar 2022 16:23:18 +0200
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Subject: Re: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as
 a perf event clock
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        kvm@vger.kernel.org, H Peter Anvin <hpa@zytor.com>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Leo Yan <leo.yan@linaro.org>, jgross@suse.com,
        sdeep@vmware.com, pv-drivers@vmware.com, pbonzini@redhat.com,
        seanjc@google.com, kys@microsoft.com, sthemmin@microsoft.com,
        virtualization@lists.linux-foundation.org,
        Andrew.Cooper3@citrix.com, christopher.s.hall@intel.com
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From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
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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.