linux-kernel - Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT

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Date:	Mon, 09 Mar 2015 10:44:23 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <vda.linux@...glemail.com>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	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>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On 03/09/2015 09:44 AM, Linus Torvalds wrote:
> 
> And remember: those zero-cost out-of-order branches turn quite
> expensive if they *ever* mispredict. Even a 5% mispredict rate is
> likely to mean "it's better to have a data dependency chain".
> 
> So it could easily go either way. I'm not convinced the old code is bad at all.
> 

I'm inclined to side with Linus here.  I'm hesitant to change this based
on pure speculation.

To answer Andy's question: I do believe we need espfix for V86 mode as well.

	-hpa


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