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Date: Thu, 18 Sep 2008 16:38:02 -0700
From: Avi Kivity <avi@redhat.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Nick Piggin <nickpiggin@yahoo.com.au>, Hugh Dickens <hugh@veritas.com>,
       Linux Memory Management List <linux-mm@kvack.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Avi Kivity <avi@qumranet.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Rik van Riel <riel@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: Populating multiple ptes at fault time
References: <48D142B2.3040607@goop.org> <48D17E75.80807@redhat.com> <48D1851B.70703@goop.org> <48D18919.9060808@redhat.com> <48D18C6B.5010407@goop.org> <48D2B970.7040903@redhat.com> <48D2D3B2.10503@goop.org>
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Jeremy Fitzhardinge wrote:
> 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?  

I believe so.  The cpu won't cache tlb entries with the A bit clear 
(much like the shadow code), and will rmw the pte on first access.

> Can
> speculative accesses set it?  

Yes, but don't abuse this.

>> 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
>   

Not the RSS (that's pte.present pages) but the working set (aka active 
list).

> 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).
>   

It would have to be a new lazy mode, not the existing one, I think.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.