Re: Populating multiple ptes at fault time

[Avi Kivity <avi@redhat.com>]

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.

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