linux-kernel - Re: [PATCH 1/4] x86/fpu: Add kernel_fpu_begin

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Message-ID: <20210119110834.GH27433@zn.tnic>
Date:   Tue, 19 Jan 2021 12:08:34 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Krzysztof Mazur <krzysiek@...lesie.net>,
        Krzysztof Olędzki <ole@....pl>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 1/4] x86/fpu: Add kernel_fpu_begin_mask() to selectively
 initialize state

Just nitpicks:

On Sun, Jan 17, 2021 at 10:20:38PM -0800, Andy Lutomirski wrote:
> Currently, requesting kernel FPU access doesn't distinguish which parts of
> the extended ("FPU") state are needed.  This is nice for simplicity, but
> there are a few cases in which it's suboptimal:
> 
>  - The vast majority of in-kernel FPU users want XMM/YMM/ZMM state but do
>    not use legacy 387 state.  These users want MXCSR initialized but don't
>    care about the FPU control word.  Skipping FNINIT would save time.
>    (Empirically, FNINIT is several times slower than LDMXCSR.)
> 
>  - Code that wants MMX doesn't want need MXCSR or FCW initialized.

"want/need" ?

>    _mmx_memcpy(), for example, can run before CR4.OSFXSR gets set, and
>    initializing MXCSR will fail.

"... because LDMXCSR generates an #UD when the aforementioned CR4 bit is
not set."

>  - Any future in-kernel users of XFD (eXtended Feature Disable)-capable
>    dynamic states will need special handling.
> 
> This patch adds a more specific API that allows callers specify exactly

s/This patch adds/Add/

> what they want.

-- 
Regards/Gruss,
    Boris.

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