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

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Date:	Tue, 30 Sep 2014 22:24:35 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Sebastian Lackner <sebastian@...-team.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.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 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.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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