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

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Message-ID: <CA+55aFySbPjeCgetxeSQPSP-h+yKnrHPcTGuoorAHJbCj53-8A@mail.gmail.com>
Date:	Mon, 27 Apr 2015 11:14:15 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

On Mon, Apr 27, 2015 at 9:40 AM, Borislav Petkov <bp@...en8.de> wrote:
>
> Either way, the NOPs-version is faster and I'm running the test with the
> F16h-specific NOPs to see how they perform.

Btw, please don't use the "more than three 66h overrides" version.
Sure, that's what the optimization manual suggests if you want
single-instruction decode for all sizes up to 15 bytes, but I think
we're better off with the two-nop case for sizes 12-15) (4-byte nop
followed by 8-11 byte nop).

Because the "more than three 66b prefixes" really performs abysmally
on some cores, iirc.

                       Linus
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