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

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Message-ID: <5383B65F.6030602@zytor.com>
Date:	Mon, 26 May 2014 14:47:11 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"Yu, Fenghua" <fenghua.yu@...el.com>
CC:	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 05/26/2014 02:40 PM, Andy Lutomirski wrote:
>>>
>>> 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.
> 

XSAVEOPT and XSAVES would be the same in this case.  Keep in mind, too,
that relatively few system calls result in context switches, which is
the only time this ends up mattering.

	-hpa


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