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

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Message-ID: <ef327682-7e9f-5652-1664-3e201ac38f60@intel.com>
Date:   Mon, 22 Jun 2020 11:38:04 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>
Cc:     X86 ML <x86@...nel.org>, jpa@...nelbug.mail.kapsi.fi,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/fpu: Reset MXCSR to default in kernel_fpu_begin()

On 6/22/20 11:33 AM, Andy Lutomirski wrote:
> Suppose you do:
> 
> double x = 1.0;
> 
> kernel_fpu_begin();
> 
> x += 2.0;
> 
> We want to make sure that GCC puts things in the right order.  I
> suppose that even a memory clobber is insufficient here, though.

Even with CONFIG_PREEMPT disabled, we still have:

	#define preempt_disable()                       barrier()

I don't see us supporting preemptible kernel_fpu regions any time soon,
so shouldn't this be sufficient now and for a long time?