linux-kernel - Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager fpu mode, too

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Date:	Tue, 13 Jan 2015 15:43:46 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Borislav Petkov <bp@...e.de>, Oleg Nesterov <oleg@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 06/11] x86,fpu: lazily skip fpu restore with eager
 fpu mode, too

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On 01/13/2015 12:11 PM, Andy Lutomirski wrote:
> On Sun, Jan 11, 2015 at 1:46 PM,  <riel@...hat.com> wrote:

>> diff --git a/arch/x86/include/asm/fpu-internal.h
>> b/arch/x86/include/asm/fpu-internal.h index 4db8781..a5a40c7
>> 100644 --- a/arch/x86/include/asm/fpu-internal.h +++
>> b/arch/x86/include/asm/fpu-internal.h @@ -435,13 +435,9 @@ static
>> inline void switch_fpu_prepare(struct task_struct *old, struct
>> task_struc old->thread.fpu.last_cpu = ~0; if (preload) { 
>> new->thread.fpu_counter++; -                       if
>> (!use_eager_fpu() && fpu_lazy_restore(new, cpu)) -
>> /* XXX: is this safe against ptrace??? */ -
>> __thread_fpu_begin(new); -                       else { +
>> set_thread_flag(TIF_LOAD_FPU); +                       if
>> (!fpu_lazy_restore(new, cpu)) prefetch(new->thread.fpu.state);
> 
> Is this prefetch still worth keeping?  Wouldn't prefetching the
> fpu effectively require grabbing more than one cacheline anyway?

Probably not. I'll drop it, for more code simplification.

- -- 
All rights reversed
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