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