linux-kernel - Re: [PATCH v2 2/4] x86/mmx: Use KFPU

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Date:   Wed, 20 Jan 2021 19:18:02 +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 v2 2/4] x86/mmx: Use KFPU_387 for MMX string operations

On Tue, Jan 19, 2021 at 09:39:00AM -0800, Andy Lutomirski wrote:
> The default kernel_fpu_begin() doesn't work on systems that support XMM but
> haven't yet enabled CR4.OSFXSR.  This causes crashes when _mmx_memcpy() is
> called too early because LDMXCSR generates #UD when the aforementioned bit
> is clear.
> 
> Fix it by using kernel_fpu_begin_mask(KFPU_387) explicitly.
> 
> Fixes: 7ad816762f9b ("x86/fpu: Reset MXCSR to default in kernel_fpu_begin()")
> Reported-by: Krzysztof Mazur <krzysiek@...lesie.net>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>

Cc: <stable@...r.kernel.org> I guess.

> ---
>  arch/x86/lib/mmx_32.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/lib/mmx_32.c b/arch/x86/lib/mmx_32.c
> index 4321fa02e18d..2a6ad7aa148a 100644
> --- a/arch/x86/lib/mmx_32.c
> +++ b/arch/x86/lib/mmx_32.c
> @@ -26,6 +26,16 @@
>  #include <asm/fpu/api.h>
>  #include <asm/asm.h>
>  
> +/*
> + * For MMX, we use KFPU_387.  MMX instructions are not affected by MXCSR,
> + * but both AMD and Intel documentation states that even integer MMX
> + * operations will result in #MF if an exception is pending in FCW.
> + *
> + * We don't need EMMS afterwards because, after we call kernel_fpu_end(),
> + * any subsequent user of the 387 stack will reinitialize it using
> + * KFPU_387.

Please use passive voice and convert the "we" to something impersonal.
For example:

"Use KFPU_387 for MMX. MMX instructions are not affected by MXCSR, but
both AMD and Intel documentation states that even integer MMX operations
will result in #MF if an exception is pending in FCW.

EMMS afterwards is not needed because, after kernel_fpu_end(), any
subsequent user of the 387 stack will reinitialize it using KFPU_387."

Voila, de-we-fied!

:-)

-- 
Regards/Gruss,
    Boris.

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