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

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Date:   Mon, 25 Apr 2022 08:30:55 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        H Peter Anvin <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>,
        "jgross@...e.com" <jgross@...e.com>,
        "sdeep@...are.com" <sdeep@...are.com>,
        "pv-drivers@...are.com" <pv-drivers@...are.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "sthemmin@...rosoft.com" <sthemmin@...rosoft.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Andrew.Cooper3@...rix.com" <Andrew.Cooper3@...rix.com>,
        "Hall, Christopher S" <christopher.s.hall@...el.com>
Subject: Re: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as
 a perf event clock

On 14/03/22 13:50, Adrian Hunter wrote:
> On 08/03/2022 23:06, Hall, Christopher S wrote:
>> Adrian Hunter wrote:
>>> 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.
>>
>> Translating between ART and the guest TSC can be a difficult problem and ART software
>> support is disabled by default in a VM.
>>
>> There are two major issues translating ART to TSC in a VM:
>>
>> The range of the TSC scaling field in the VMCS is much larger than the range of values
>> that can be represented using CPUID[15H], i.e., it is not possible to communicate this
>> to the VM using the current CPUID interface. The range of scaling would need to be
>> restricted or another para-virtualized method - preferably OS/hypervisor agnostic - to
>> communicate the scaling factor to the guest needs to be invented.
>>
>> TSC offsetting may also be a problem. The VMCS TSC offset must be discoverable by the
>> guest. This can be done via TSC_ADJUST MSR. The offset in the VMCS and the guest
>> TSC_ADJUST MSR must always be equivalent, i.e. a write to TSC_ADJUST in the guest
>> must be reflected in the VMCS and any changes to the offset in the VMCS must be
>> reflected in the TSC_ADJUST MSR. Otherwise a para-virtualized method must
>> be invented to communicate an arbitrary VMCS TSC offset to the guest.
>>
> 
> In my view it is reasonable for perf to support TSC as a perf clock in any case
> because:
> 	a) it allows users to work entirely with TSC if they wish
> 	b) other kernel performance / debug facilities like ftrace already support TSC
> 	c) the patches to add TSC support are relatively small and straight-forward
> 
> May we have support for TSC as a perf event clock?

Any update on this?