linux-kernel - Re: [PATCH] perf/x86: KVM: Disable vPMU support on hybrid CPUs (host PMUs)

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Message-ID: <Y9j0kwYiqYdQpx28@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Jan 2023 11:59:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jianfeng Gao <jianfeng.gao@...el.com>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH] perf/x86: KVM: Disable vPMU support on hybrid CPUs (host
 PMUs)

On Tue, Jan 24, 2023 at 10:31:00AM -0500, Liang, Kan wrote:

> Yes, and also the CPUID leaf 0x23H support to enumerate the PMU features
> of each types.

Note that this is not enough or even useful. There is nothing that stops
a vCPU from migrating between types at every instruction. There simply
is no relation between a vCPU and a type, so knowing what a type does or
does not support is useless information.