linux-kernel - Re: [PATCH 2/3] x86, fpu: don't abuse ->has_fpu in __kernel_fpu

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Message-ID: <20150116155445.GA7249@redhat.com>
Date:	Fri, 16 Jan 2015 16:54:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Suresh Siddha <sbsiddha@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	matt.fleming@...el.com, bp@...e.de, pbonzini@...hat.com,
	tglx@...utronix.de, luto@...capital.net
Subject: Re: [PATCH 2/3] x86, fpu: don't abuse ->has_fpu in
	__kernel_fpu_{begin,end}()

On 01/15, Rik van Riel wrote:
>
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> On 01/15/2015 02:20 PM, Oleg Nesterov wrote:
> > b/arch/x86/kernel/i387.c @@ -81,9 +81,7 @@ void
> > __kernel_fpu_begin(void) this_cpu_write(in_kernel_fpu, true);
> >
> > if (__thread_has_fpu(me)) { -		__thread_clear_has_fpu(me);
>
> I will put that line back in the patch series that defers the
> loading of FPU state until the switch to user space,

I think this needs more discussion.

Firstly, __thread_clear_has_fpu() should go into __kernel_fpu_end(), it
should replace restore_fpu_checking().

But does your series actually need this change? Correctness-wise this is
not needed (afaics). Performance-wise I am not sure, kernel_fpu_begin()
is unlikely event. Plus I am not sure this is a win (in general), but I
can be easily wrong.

> but I
> guess we can go either way for now...

Yes, this should not really conflict with your changes in any case.

Given that you acked 1/3 and 3/3, perhaps you can ack this one as well?

Oleg.

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