linux-kernel - Re: [PATCH v2 1/4] x86/fpu: Add kernel_fpu_begin

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Message-Id: <F60CEFE2-22A4-405A-9BBC-7D84AFDA07C5@amacapital.net>
Date:   Wed, 20 Jan 2021 04:19:24 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 v2 1/4] x86/fpu: Add kernel_fpu_begin_mask() to selectively initialize state


> On Jan 20, 2021, at 3:53 AM, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Tue, Jan 19, 2021 at 09:38:59AM -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 or need MXCSR initialized.
>>   _mmx_memcpy(), for example, can run before CR4.OSFXSR gets set, and
>>   initializing MXCSR will fail.
>> 
>> - 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
>> what they want.
> 
> Same nitpicks:
> 
> https://lkml.kernel.org/r/20210119110834.GH27433@zn.tnic

I would have sworn I fixed those.  Sorry!

> 
> -- 
> Regards/Gruss,
>    Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette