linux-kernel - Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush

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Date:	Sun, 15 Mar 2015 18:36:20 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in
 flush_thread()

On Sat, Mar 14, 2015 at 03:48:16PM +0100, Oleg Nesterov wrote:
> On 03/13, Borislav Petkov wrote:
> > On Fri, Mar 13, 2015 at 05:26:54PM +0100, Oleg Nesterov wrote:
> > > > One example where drop_init_fpu() seems to make sense is
> > > > __kernel_fpu_end(): kernel is done with FPU and current was using the
> > > > FPU prior so let's restore it for the eagerfpu case.
> > >
> > > No, no, this is another case or I misunderstood you.
> > >
> > > __kernel_fpu_end() needs to restore FPU from current's fpu->state exactly
> > > because current used FPU prior. And that state was saved by __save_init_fpu()
> > > in __kernel_fpu_begin().
> >
> > That's exactly what I mean. See: "... kernel is done with FPU and current was
> > using the FPU prior..."
> 
> Yes, but my point was that this is why we can _not_ use drop_init_fpu() in
> __kernel_fpu_end().

Hmm, now I'm confused. So __kernel_fpu_end() says kernel finished using
the FPU and we need to do the following:

* current has the FPU => let's restore it. If there was an error doing
that, we do drop_init, i.e. restore init_xstate in the eager case and
otherwise we just drop it. So that makes perfect sense to me.

* otherwise, current didn't have the FPU, we simply set CR0.TS in the
non-eager case so that we can fault on the next use of an FPU insn.

To address your comment from earlier:

> > > __kernel_fpu_end() needs to restore FPU from current's fpu->state exactly
> > > because current used FPU prior. And that state was saved by __save_init_fpu()
> > > in __kernel_fpu_begin().

And we do that:

void __kernel_fpu_end():

...

        if (__thread_has_fpu(me)) {
                if (WARN_ON(restore_fpu_checking(me)))

restore_fpu_checking(current) does try to restore fpu->state and it does
drop_init_fpu() only if it failed.

Ok, now you tell me what I'm missing :)

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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