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

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Message-ID: <20150427184730.GH28871@pd.tnic>
Date:	Mon, 27 Apr 2015 20:47:30 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	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>,
	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 11:12:05AM -0700, Linus Torvalds wrote:
> So if one or two cycles in this code doesn't matter, then why are we
> adding alternate instructions just to avoid a few ALU instructions and
> a conditional branch that predicts perfectly? And if it does matter,
> then the 6-byte option looks clearly better..

You know what? I haven't even measured the tree *without* Denys'
stricter RCX canonical-ness patch. All numbers so far are from 4.0+ with
tip/master ontop which has Denys' patch.

And I *should* measure once with plain 4.1-rc1 and then with Denys'
patch ontop to see whether this all jumping-thru-alternatives-hoops is
even worth it.

If nothing else, it was a nice exercise. :-)

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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