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Date: Mon, 03 Nov 2014 14:17:28 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>, Jan Beulich <jbeulich@suse.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sebastian Lackner <sebastian@fds-team.de>,
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        Chuck Ebbert <cebbert.lkml@gmail.com>, Anish Bhatt <anish@chelsio.com>,
        Thomas Gleixner <tglx@linutronix.de>, Oleg Nesterov <oleg@redhat.com>
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Subject: Re: [tip:x86/asm] sched/x86_64: Don't save flags on context switch
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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@zytor.com> wrote:
>> Commit-ID:  2c7577a7583747c9b71f26dced7f696b739da745
>> Gitweb:     http://git.kernel.org/tip/2c7577a7583747c9b71f26dced7f696b739da745
>> Author:     Andy Lutomirski <luto@amacapital.net>
>> AuthorDate: Wed, 1 Oct 2014 11:28:25 -0700
>> Committer:  Ingo Molnar <mingo@kernel.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