linux-kernel - Re: [PATCH 1/2] x86/fpu: Reset MXCSR to default in kernel_fpu

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Message-ID: <20200622183657.GH20421@lakka.kapsi.fi>
Date:   Mon, 22 Jun 2020 21:36:57 +0300
From:   Petteri Aimonen <jpa@...nelbug.mail.kapsi.fi>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

> We want to make sure that GCC puts things in the right order.  I
> suppose that even a memory clobber is insufficient here, though.

amdgpu worked around that by using a noinline function:
https://github.com/torvalds/linux/commit/59dfb0c64d3853d20dc84f4561f28d4f5a2ddc7d#diff-a82b8ab0e6b4f9abfd3344d1427d765f

If there is something that would help, it would have to be an
attribute added to kernel_fpu_begin() in the header. Any barriers
inside the function will be invisible to code generation in other
compilation units.

--
Petteri