Re: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for non-present faults

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 05/15/2014 12:22 PM, Keir Fraser wrote:
>>
>> Are we chasing hardware errata here?  Or did someone go off and *assume*
>> that the x86 hardware architecture work a certain way?  Or is there
>> something way more subtle going on?
> 
> See Intel Developer's Manual Vol 3 Section 4.10.4.3, 3rd bullet... This
> is expected behaviour, probably to make copy-on-write faults faster.
> 

Hm, yes.  My memory of this comes from before these formal rules were
written down... I guess there is some wiggle room in there, presumably
as you say, for performance reasons (or implementation leeway, which is
another way to say performance.)

This does make a P bit switch architecturally different from W or NX, so
I'm okay with that, but I would like the patch adjusted in the following
ways:

1. Put in an explicit comment about the architectural difference
   between the P bit on one hand and an W and NX on the other; an SDM
   reference is good, and *why* this makes the specific filtering
   correct.

2. Please use the standard format for multiline comments;

        /*
         * blah
         * blah
         */

With that this should be okay.

        -hpa


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