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SubjectRe: [PATCH V2 03/11] perf/x86: Add support for TSC in nanoseconds as a perf event clock
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On 07.03.22 10:50, Peter Zijlstra wrote:
> On Fri, Mar 04, 2022 at 08:27:45PM +0200, Adrian Hunter wrote:
>> On 04/03/2022 15:41, Peter Zijlstra wrote:
>>> On Mon, Feb 14, 2022 at 01:09:06PM +0200, Adrian Hunter wrote:
>>>> Currently, when Intel PT is used within a VM guest, it is not possible to
>>>> make use of TSC because perf clock is subject to paravirtualization.
>>>
>>> Yeah, so how much of that still makes sense, or ever did? AFAIK the
>>> whole pv_clock thing is utter crazy. Should we not fix that instead?
>>
>> Presumably pv_clock must work with different host operating systems.
>> Similarly, KVM must work with different guest operating systems.
>> Perhaps I'm wrong, but I imagine re-engineering time virtualization
>> might be a pretty big deal, far exceeding the scope of these patches.
>
> I think not; on both counts. That is, I don't think it's going to be
> hard, and even it if were, it would still be the right thing to do.
>
> We're not going to add interface just to work around a known broken
> piece of crap just because we don't want to fix it.
>
> So I'm thinking we should do the below and simply ignore any paravirt
> sched clock offered when there's ART on.
>
> ---
> 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.


Juergen
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