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

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Message-ID: <20150225105020.GF554@gmail.com>
Date:	Wed, 25 Feb 2015 11:50:20 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>, X86 ML <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs


* Borislav Petkov <bp@...en8.de> wrote:

> On Tue, Feb 24, 2015 at 04:07:07PM -0800, Andy Lutomirski wrote:
>
> > I'd prefer a different partial solution: encourage 
> > everyone to clear the xstate before making syscalls 
> > (using e.g. vzeroall).  In fact, maybe user code should 
> > aggressively clear newly-unused xstate.
> 
> We don't trust userspace.

We certainly don't, but in this case it's a performance 
optimization detail: if user-space clears unused xstate in 
a way that the CPU recognizes it (for example VZEROALL) 
then it might get faster context switches.

Thanks,

	Ingo
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