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

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Message-ID: <53751AC8.6040902@zytor.com>
Date:	Thu, 15 May 2014 12:51:36 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Keir Fraser <keir.xen@...il.com>
CC:	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH 7/9] x86: skip check for spurious faults for
 non-present faults

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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