linux-kernel - Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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Date:	Sun, 22 Feb 2015 20:45:37 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Borislav Petkov <bp@...en8.de>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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On 02/22/2015 06:06 AM, Borislav Petkov wrote:
> On Sat, Feb 21, 2015 at 06:18:01PM -0800, Andy Lutomirski wrote:
>> That's true.  The question is whether there are enough of them,
>> and whether twiddling TS is fast enough, that it's worth it.
> 
> Yes, and let me make it clear what I'm trying to do here: I want to
> make sure that eager FPU handling (both allocation and switching -
> and no, I'm not confusing the concepts) *doesn't* *hurt* any
> relevant workload. If it does, then we'll stop wasting time right
> there.
> 
> But(!), if the CR0.TS lazy dance turns out to be really slow and
> the eager handling doesn't bring a noticeable slowdown, in
> comparison, we should consider doing the eager thing by default.
> After running a lot more benchmarks, of course.
> 
> Which brings me to the main reason why we're doing this: code 
> simplification. If we switch to eager, we'll kill a lot of
> non-trivial code and the FPU handling in the kernel becomes dumb
> and nice again.

Currently the FPU handling does something really dumb for
KVM VCPU threads.  Specifically, every time we enter a
KVM guest, we save the userspace FPU state of the VCPU
thread, and every time we leave the KVM guest, we load
the userspace FPU state of the VCPU thread.

This is done for a thread that hardly ever exits to
userspace, and generally just switches between kernel
and guest mode.

The reason for this acrobatics is that at every
context switch time, the userspace FPU state is
saved & loaded.

I am working on a patch series to avoid that needless
FPU save & restore, by moving the point at which the
user FPU state is loaded out to the point where we
return to userspace, in do_notify_resume.

One implication of this is that in kernel mode, we
can no longer just assume that the user space FPU
state is always loaded, and we need to check for that
(like the lazy FPU code does today).  I would really
like to keep that code around, for obvious reasons :)

- -- 
All rights reversed
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