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

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Message-ID: <542C2916.7010500@zytor.com>
Date:	Wed, 01 Oct 2014 09:17:26 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Sebastian Lackner <sebastian@...-team.de>, X86 ML <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anish Bhatt <anish@...lsio.com>,
	Ingo Molnar <mingo@...hat.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 10/01/2014 09:04 AM, Andy Lutomirski wrote:
> 
> Agner Fog's tables for Sandy Bridge have 9 uops for popf and
> reciprocal throughput 18.  sti isn't listed for Sandy Bridge or
> anything similar, but cld is 3 uops with reciprocal throughput 4.
> Also, popf accesses rsp, and the sysenter code is very heavy on stack
> manipulation.
> 

It does a stack operation.  Newer CPUs optimize stack accesses pretty
heavily.  That doesn't mean back-to-back push/pop are all that
optimized, I wonder if it would help separating them.  popf is unlikely
to ever be all that fast.

	-hpa

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