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

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Message-ID: <CALCETrWM2wVynp1rrip9AWtNYDzjC4M8JqKR82ew5dpyrTWMgA@mail.gmail.com>
Date:	Mon, 3 Nov 2014 13:11:51 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...capital.net>,
	Jan Beulich <jbeulich@...e.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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 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'?

--Andy
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