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:   Thu, 20 Oct 2022 10:59:48 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     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>,
        "H. Peter Anvin" <hpa@...or.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 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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette