linux-kernel - RE: [PATCH 0/15] x86/xsaves: Optimize xstate context switch by xsaves/xrstors

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Date:	Mon, 26 May 2014 20:13:50 +0000
From:	"Yu, Fenghua" <fenghua.yu@...el.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>
CC:	linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: RE: [PATCH 0/15] x86/xsaves: Optimize xstate context switch by
 xsaves/xrstors

> From: Andy Lutomirski [mailto:luto@...capital.net]
> On 05/26/2014 10:01 AM, Fenghua Yu wrote:
> > From: Fenghua Yu <fenghua.yu@...el.com>
> >
> > With ever growing extended state registers (xstate) on x86 processors,
> kernel
> > needs to cope with issue of growing memory space occupied by xstate.
> The xsave
> > area is holding more and more xstate registers, growing from legacy
> FP and
> > SSE to AVX, AVX2, AVX-512, MPX, and Intel PT.
> >
> > The recently introduced compacted format of xsave area saves xstates
> only
> > for enabled states. This patch set saves the xsave area space per
> process
> > in compacted format by xsaves/xrstors instructions.
> 
> Are we going to want to encourage userspace to do something like
> sticking vzeroupper right before each syscall to make any
> xsaves/xrestores faster?

This patch set allow compacted format in kernel and standard format
in user space. This works fine for both kernel and user application.

As said in the patch 9/15, there would be two issues for user applications
to use xsaves/xrstors: one is modified optimization doesn't work for
user applications and the other one is legacy applications don't
know compacted format of xsave area.

So I don't see we need xsaves/xrstors in user space. But I could be
wrong.

Thanks.

-Fenghua
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