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

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Message-ID: <YAV9TBMZ+Xs9ielC@hirez.programming.kicks-ass.net>
Date:   Mon, 18 Jan 2021 13:21:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
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

On Sun, Jan 17, 2021 at 10:20:38PM -0800, Andy Lutomirski wrote:

>  - Code that wants MMX doesn't want need MXCSR or FCW initialized.
>    _mmx_memcpy(), for example, can run before CR4.OSFXSR gets set, and
>    initializing MXCSR will fail.

> +#define KFPU_MMX	0		/* nothing gets initialized */

This... why is that correct?