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

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Date:	Mon, 09 Mar 2015 12:26:49 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	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 12:13 PM, Andy Lutomirski wrote:
> On Mon, Mar 9, 2015 at 10:44 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> 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.
>>
> 
> I think we don't.  Did I screw up my test?
> 

I don't see how your test executes V86 mode code at all, since there
seems to be nothing mapped at the bottom of memory?

	-hpa


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