linux-kernel - Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu

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Date:   Sat, 22 Oct 2022 14:08:37 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Borislav Petkov <bp@...en8.de>,
        Maxim Levitsky <mlevitsk@...hat.com>
CC:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Jane Malalane <jane.malalane@...rix.com>,
        Kees Cook <keescook@...omium.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jiri Olsa <jolsa@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-perf-users@...r.kernel.org,
        "open list:CRYPTO API" <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] perf/x86/intel/lbr: use setup_clear_cpu_cap instead of clear_cpu_cap

On October 20, 2022 1:59:48 AM PDT, Borislav Petkov <bp@...en8.de> wrote:
>On Wed, Sep 28, 2022 at 01:49:34PM +0300, Maxim Levitsky wrote:
>> Patch 5 is the main fix - it makes the kernel to be tolerant to a
>> broken CPUID config (coming hopefully from hypervisor), where you have
>> a feature (AVX2 in my case) but not a feature on which this feature
>> depends (AVX).
>
>I really really don't like it when people are fixing the wrong thing.
>
>Why does the kernel need to get fixed when something else can't get its
>CPUID dependencies straight? I don't even want to know why something
>would set AVX2 without AVX?!?!
>
>Srsly.
>
>Some of your other bits look sensible and I'd take a deeper look but
>this does not make any sense. This is a hypervisor problem - not a
>kernel one.
>
>Thx.
>

Yes, this is utterly nonsensical and it will break user space applications left, right, and center.