linux-kernel - Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

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Date:	Wed, 01 Oct 2014 08:19:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Sebastian Lackner <sebastian@...-team.de>
CC:	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Anish Bhatt <anish@...lsio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace

On 09/30/2014 10:24 PM, Andy Lutomirski wrote:
> On Tue, Sep 30, 2014 at 10:09 PM, Sebastian Lackner
> <sebastian@...-team.de> wrote:
>>> +     testl $X86_EFLAGS_NT,EFLAGS(%rsp)       /* saved EFLAGS match cpu */
>>> +     jz 1f
>>> +     pushq_cfi $(X86_EFLAGS_IF|X86_EFLAGS_FIXED)
>>> +     popfq_cfi
>>> +1:
>>> +
>>
>> Do you think it makes sense to change the order here, so that no jump happens if
>> NT is not set (which happens a bit more often, than the other way round)? Just a
>> guess though, haven't measured if pipeline effects have such a big influence in this
>> case. ;)
>>
> 
> It should be immeasurable in a tight loop, since it will predict
> correctly almost every time.  And, unless cfi state works across
> .pushsection (does it?), getting the cfi annotations right will be
> more complicated.
> 

It does, actually... otherwise it would be almost impossible to use in a
lot of cases.

	-hpa


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