linux-kernel - Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics event

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Message-ID: <20231011142003.GF19999@noisy.programming.kicks-ass.net>
Date:   Wed, 11 Oct 2023 16:20:03 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Mingwei Zhang <mizhang@...gle.com>, Ingo Molnar <mingo@...nel.org>,
        Dapeng Mi <dapeng1.mi@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Like Xu <likexu@...cent.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>, kvm@...r.kernel.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Zhenyu Wang <zhenyuw@...ux.intel.com>,
        Zhang Xiong <xiong.y.zhang@...el.com>,
        Lv Zhiyuan <zhiyuan.lv@...el.com>,
        Yang Weijiang <weijiang.yang@...el.com>,
        Dapeng Mi <dapeng1.mi@...el.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Patch v4 07/13] perf/x86: Add constraint for guest perf metrics
 event

On Wed, Oct 04, 2023 at 02:50:46PM -0700, Sean Christopherson wrote:

> Thinking about this more, what if we do a blend of KVM's FPU swapping and debug
> register swapping?
> 
>   A. Load guest PMU state in vcpu_enter_guest() after IRQs are disabled
>   B. Put guest PMU state (and load host state) in vcpu_enter_guest() before IRQs
>      are enabled, *if and only if* the current CPU has one or perf events that
>      wants to use the hardware PMU
>   C. Put guest PMU state at vcpu_put()
>   D. Add a perf callback that is invoked from IRQ context when perf wants to
>      configure a new PMU-based events, *before* actually programming the MSRs,
>      and have KVM's callback put the guest PMU state
> 

No real objection, but I would suggest arriving at that solution by
first building the simple-stupid thing and then making it more
complicated in additinoal patches. Hmm?