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Date: Wed, 01 Oct 2014 09:17:26 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Sebastian Lackner <sebastian@fds-team.de>, X86 ML <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
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	Ingo Molnar <mingo@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Chuck Ebbert <cebbert.lkml@gmail.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] x86_64,entry: Filter RFLAGS.NT on entry from userspace
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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