linux-kernel - Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until switch to userspace

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Message-ID: <1475368956.21644.15.camel@redhat.com>
Date:   Sat, 01 Oct 2016 20:42:36 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH RFC 2/5] x86,fpu: delay FPU register loading until
 switch to userspace

On Sat, 2016-10-01 at 16:44 -0700, Andy Lutomirski wrote:
> On Sat, Oct 1, 2016 at 1:31 PM,  <riel@...hat.com> wrote:
> > 
> > 
> > +static inline void switch_fpu_finish(void)
> >  {
> > +       set_thread_flag(TIF_LOAD_FPU);
> >  }
> 
> I can imagine this causing problems with kernel code that accesses
> current's FPU state, e.g. get_xsave_field_ptr(). 

That makes me wonder, what test programs do people have
to verify the correctness of the FPU switching code?

I have a few floating point benchmarks that check
the results for correctness, but nothing that adds in
signals, for example.

What do people use?

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