linux-kernel - Re: [PATCH v3 05/17] KVM: x86/pmu: Reprogram guest PEBS event to emulate guest PEBS counter

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Message-ID: <38e774f5-81d6-4853-cbb9-d4b7811e65db@intel.com>
Date:   Fri, 15 Jan 2021 21:53:22 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        kvm@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi@...stfloor.org>,
        Kan Liang <kan.liang@...ux.intel.com>, wei.w.wang@...el.com,
        luwei.kang@...el.com, linux-kernel@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v3 05/17] KVM: x86/pmu: Reprogram guest PEBS event to
 emulate guest PEBS counter

On 2021/1/15 19:33, Peter Zijlstra wrote:
> On Mon, Jan 04, 2021 at 09:15:30PM +0800, Like Xu wrote:
>> When a guest counter is configured as a PEBS counter through
>> IA32_PEBS_ENABLE, a guest PEBS event will be reprogrammed by
>> configuring a non-zero precision level in the perf_event_attr.
>>
>> The guest PEBS overflow PMI bit would be set in the guest
>> GLOBAL_STATUS MSR when PEBS facility generates a PEBS
>> overflow PMI based on guest IA32_DS_AREA MSR.
>>
>> The attr.precise_ip would be adjusted to a special precision
>> level when the new PEBS-PDIR feature is supported later which
>> would affect the host counters scheduling.
> This seems like a random collection of changes, all required, but
> loosely related.

Yes, these changes are made in the KVM context, and
they are all necessary to emulate basic PEBS hw behavior.

>
>> The guest PEBS event would not be reused for non-PEBS
>> guest event even with the same guest counter index.

Let me add more KVM context here,

we would create a perf_event for a normal non-PEBS counter
and we reuse the same perf_event from time to time as much as possible
instead of "create + destroy" new perf_event.

So when a normal counter is configured for PEBS,
the original perf_event would not be reused and
a new PEBS perf_event is created, vice verse.

> /me rolls eyes at the whole destroy+create nonsense...

I absolutely agree that cross-domain development
may make maintainers' eyes uncomfortable.

My on-demand explanation is always online
if you fire more questions on this patch set.