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Date:   Tue, 7 Apr 2020 07:20:03 -0700
From:   Andi Kleen <andi@firstfloor.org>
To:     Keno Fischer <keno@juliacomputing.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andi Kleen <andi@firstfloor.org>,
        Kyle Huey <khuey@kylehuey.com>,
        Robert O'Callahan <robert@ocallahan.org>
Subject: Re: [RFC PATCH v2] x86/arch_prctl: Add ARCH_SET_XCR0 to set XCR0
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> Thank you in advance for any review comments - I thought the comments
> on v1 were quite helpful. I understand this is a tall ask for mainline
> inclusion, but the feature is just too useful for me to give up ;).

> [1] https://lkml.org/lkml/2018/6/16/134

The rationale from that post should have been in this description.

You can already do what you want using the clearcpuid= boot flags using the
infrastructure in [1], which is in newer kernels.

So to disable AVX512 you would use clearcpuid=304 and the AVX feature
set should be the same as Haswell. To disable AVX2 you would use
clearcpuid=293 and you get the same feature set as Sandy Bridge.

A boot option is not as convenient as a run time setting, but has a lot
less weird corner cases.

-Andi

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/x86/kernel/cpu/cpuid-deps.c?id=0b00de857a648dafe7020878c7a27cf776f5edf4

-Andi