linux-kernel - Re: [tip:x86/asm] sched/x86_64: Don't save flags on context switch

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Date:	Mon, 03 Nov 2014 14:17:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Jan Beulich <jbeulich@...e.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sebastian Lackner <sebastian@...-team.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	Anish Bhatt <anish@...lsio.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>
CC:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] sched/x86_64: Don't save flags on context switch

On 11/03/2014 01:11 PM, Andy Lutomirski wrote:
> On Tue, Oct 28, 2014 at 4:14 AM, tip-bot for Andy Lutomirski
> <tipbot@...or.com> wrote:
>> Commit-ID:  2c7577a7583747c9b71f26dced7f696b739da745
>> Gitweb:     http://git.kernel.org/tip/2c7577a7583747c9b71f26dced7f696b739da745
>> Author:     Andy Lutomirski <luto@...capital.net>
>> AuthorDate: Wed, 1 Oct 2014 11:28:25 -0700
>> Committer:  Ingo Molnar <mingo@...nel.org>
>> CommitDate: Tue, 28 Oct 2014 11:11:30 +0100
>>
>> sched/x86_64: Don't save flags on context switch
>>
>> Now that the kernel always runs with clean flags (in particular,
>> NT is clear), there is no need to save and restore flags on
>> every context switch.
> 
> Just to make myself a little more comfortable with this...
> 
> There is one potentially relevant flag: AC.  I think this is still OK.
> If we schedule with STAC set, then we've already screwed up, I think.
> Even preempt schedules from interrupt context, so if we schedule due
> to preemption or #PF in the middle of uaccess, AC should be saved and
> cleared by whatever interrupt caused the reschedule, right?
> 
> And do we ever have TF set during a context switch?  I hope not.
> 
> Also, what's with 'jmp exit_intr' at the end of retint_kernel?  Why
> isn't that 'jmp retint_kernel'?
> 

AC is saved by interrupts and cleared by IRET.  We execute CLAC in all
the interrupt entry paths.

	-hpa


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