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, 7 Mar 2022 11:58:19 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, 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,
        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>, sdeep@...are.com,
        pv-drivers@...are.com, pbonzini@...hat.com, seanjc@...gle.com,
        kys@...rosoft.com, sthemmin@...rosoft.com,
        virtualization@...ts.linux-foundation.org,
        Andrew.Cooper3@...rix.com
Subject: Re: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as
 a perf event clock

On 07.03.22 11:38, Peter Zijlstra wrote:
> On Mon, Mar 07, 2022 at 11:06:46AM +0100, Juergen Gross 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))
>>> +		return;
>>> +
>>>    	static_call_update(pv_sched_clock, func);
>>>    }
>>>
>>
>> NAK, this will break live migration of a guest coming from a host
>> without this feature.
> 
> I thought the whole live-migration nonsense made sure to equalize crud
> like that. That is, then don't expose ART to the guest.

Oh, right. I managed to confuse host-side and guest-side usage.

Sorry for the noise.


Juergen

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