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

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Message-ID: <54B93726.4050309@redhat.com>
Date:	Fri, 16 Jan 2015 11:07:02 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...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}()

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On 01/16/2015 10:54 AM, Oleg Nesterov wrote:
> 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.

__kernel_fpu_begin() / __kernel_fpu_end() are used all the time
when running KVM guests. The KVM VCPU thread has both a user space
and a guest VCPU state. Generally the VCPU thread stays in the kernel,
and rarely (if ever) exits to user space.

>> 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?

Sure, we can figure out the cleanest thing to do for KVM later on.
Lets get these cleanups merged first.

Acked-by: Rik van Riel <riel@...hat.com>

- -- 
All rights reversed
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