Re: [PATCH 207/208] x86/fpu: Add FPU performance measurement subsystem

[Ingo Molnar]

* Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> 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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