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

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Date:	Mon, 26 May 2014 14:40:06 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	"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>,
	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

On Mon, May 26, 2014 at 1:13 PM, Yu, Fenghua <fenghua.yu@...el.com> wrote:
>> 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.

My question is purely about optimization: if userspace does a blocking
system call, will it be significantly faster if userspace zeros out as
much of the extended state as possible before doing the system call?

I think I tried this once with xsaveopt and decided that it didn't
make much of a difference.

--Andy
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