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Date: Mon, 09 Mar 2015 12:26:49 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Denys Vlasenko <vda.linux@googlemail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
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Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
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On 03/09/2015 12:13 PM, Andy Lutomirski wrote:
> On Mon, Mar 9, 2015 at 10:44 AM, H. Peter Anvin <hpa@zytor.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