Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

From: H. Peter Anvin
Date: Mon Mar 09 2015 - 15:27:28 EST

On 03/09/2015 12:13 PM, Andy Lutomirski wrote:
> On Mon, Mar 9, 2015 at 10:44 AM, H. Peter Anvin <hpa@xxxxxxxxx> 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?


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