linux-kernel - Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues

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Date:	Mon, 2 May 2016 09:28:04 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues

On 04/30/2016 12:53 AM, Ingo Molnar wrote:
> We can still use the compacted area handling instructions, because presumably 
> those are the fastest and are also the most optimized ones? But I wouldn't use 
> them to do dynamic allocation: just allocate the maximum possible FPU save area at 
> task creation time and never again worry about that detail.
> 
> Ok?

Sounds sane to me.

BTW, I hacked up your "fpu performance" to compare XSAVE vs. XSAVES:

> [    0.048347] x86/fpu: Cost of: XSAVE                       insn          :   127 cycles
> [    0.049134] x86/fpu: Cost of: XSAVES                      insn          :   113 cycles
> [    0.048492] x86/fpu: Cost of: XRSTOR                      insn          :   120 cycles
> [    0.049267] x86/fpu: Cost of: XRSTORS                     insn          :   102 cycles

So I guess we can add that to the list of things that XSAVES is good
for.  Granted, the real-world benefit is probably hard to measure
because the cache residency of the XSAVE buffer isn't as good when
_actually_ context switching, but this at least shows a small
theoretical advantage for XSAVES.