linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Message-ID: <2F6CA156-F03F-4F49-A6B9-7D1D1E1D805B@zytor.com>
Date:	Mon, 27 Apr 2015 13:14:51 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

I did a microbenchmark in user space... let's see if I can find it.

On April 27, 2015 1:03:29 PM PDT, Borislav Petkov <bp@...en8.de> wrote:
>On Mon, Apr 27, 2015 at 12:59:11PM -0700, H. Peter Anvin wrote:
>> It really comes down to this: it seems older cores from both Intel
>> and AMD perform better with 66 66 66 90, whereas the 0F 1F series is
>> better on newer cores.
>>
>> When I measured it, the differences were sometimes dramatic.
>
>How did you measure that? I should probably do the same on some newer
>AMD machines... We're using k8_nops on all AMD, even though the
>optimization manuals cite different NOPs on newer AMD families.
>
>Thanks.

-- 
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