linux-kernel - Re: Populating multiple ptes at fault time

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Message-ID: <48D2D3B2.10503@goop.org>
Date:	Thu, 18 Sep 2008 15:18:26 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickens <hugh@...itas.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: Populating multiple ptes at fault time

Avi Kivity wrote:
>> Do you need to set the A bit synchronously?  
>
> Yes, of course (if no guest cooperation).

Is the A bit architecturally guaranteed to be synchronously set?  Can
speculative accesses set it?  SDM vol 3 is a bit vague about it.

> I'll fail my own unit tests.
>
> If we add an async mode for guests that can cope, maybe this is
> workable.  I guess this is what you're suggesting.
>

Yes.  At worst Linux would underestimate the process RSS a bit
(depending on how many unsynchronized ptes you leave lying around).  I
bet there's an appropriate pvop hook you could use to force
synchronization just before the kernel actually inspects the bits
(leaving lazy mode sounds good).

    J
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