linux-kernel - Re: [PATCH v2 01/17] asm: simd context helper API

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Message-ID: <CAHmME9rL2omQxqMVCcX-jPMpcQDk8ftEn3YXWcFhS=AkVK8ZXw@mail.gmail.com>
Date:   Sat, 1 Sep 2018 14:34:01 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Andrew Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>, linux-arch@...r.kernel.org,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v2 01/17] asm: simd context helper API

On Sat, Sep 1, 2018 at 2:32 PM Andy Lutomirski <luto@...nel.org> wrote:
> I tend to think the right approach is to merge Jason's code and then
> make it better later.  Even with a totally perfect lazy FPU restore
> implementation on x86, we'll probably still need some way of dealing
> with SIMD contexts.  I think we're highly unlikely to ever a allow
> SIMD usage in all NMI contexts, for example, and there will always be
> cases where we specifically don't want to use all available SIMD
> capabilities even if we can.  For example, generating random numbers
> does crypto, but we probably don't want to do *SIMD* crypto, since
> that will force a save and restore and will probably fire up the
> AVX512 unit, and that's not worth it unless we're already using it for
> some other reason.
>
> Also, as Rik has discovered, lazy FPU restore is conceptually
> straightforward but isn't entirely trivial :)

Sounds good. I'll move ahead on this basis.