From: Ingo Molnar <mingo2.kernel.org@gmail.com>
Date: Wed, 6 May 2015 06:11:04 +0200
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
        Borislav Petkov <bp@alien8.de>, Fenghua Yu <fenghua.yu@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement
 subsystem
Message-ID: <20150506041104.GA12080@gmail.com>
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 <554916B4.1040809@linux.intel.com>
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* Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 05/05/2015 10:58 AM, Ingo Molnar wrote:
> >   x86/fpu: Cost of: XSAVE                       insn          :   104 cycles
> >   x86/fpu: Cost of: XRSTOR                      insn          :    80 cycles
> 
> Isn't there going to be pretty huge variability here depending on 
> how much state you are xsave/xrstor'ing and if the init/modified 
> optimizations are in play?

Hopefully there's such variability! :)

I thought to add measurements for that as well:

 - to see the costs of this instruction family when various xstate 
   components are in 'init state' or not

 - maybe even measure whether it can optimize based on whether things
   got changed since the last save (which the SDM kind of alludes to 
   but which I doubt the hw does)?

This initial version only measures trivial init state save/restore 
cost.

Thanks,

	Ingo
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