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

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Message-ID: <CALCETrUT1_WVjMbPZhUcX9sjFvLBZKCc2fxeJJsD940k1Ajyrw@mail.gmail.com>
Date:   Mon, 22 Jun 2020 11:40:38 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        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 Mon, Jun 22, 2020 at 11:38 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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?

That's on the wrong end of the function.  It'sL

preempt_disable();
LDMXCSR;
<-- some kind of barrier here might be nice

--Andy