Re: Populating multiple ptes at fault time

[Avi Kivity]

(potential victim cc'ed)

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
>   
> > We could work around it by having a hypercall to read and clear
> > accessed bits.  If we know the guest will only do that via the
> > hypercall, we can keep the accessed (and dirty) bits in the host, and
> > not update them in the guest at all.  Given good batching, there's
> > potential for a large win there.
> >     
>
> We added a hypercall to update just the AD bits, though it was primarily
> to update D without losing the hardware-set A bit.
>
> I don't think it would be practical to add a hypercall to read the A
> bit.  There's too much code which just assumes it can grab a pte and
> test the bit state.  There's no pv_op for reading a pte in general, and
> even if there were you'd need to have a specialized pv-op for
> specifically reading the A bit to avoid unnecessary hypercalls.
>
>   

I didn't think so much code would be interested in the accessed bit.  I 
can think of

- pte teardown (to mark the page accessed)
- scanning the active list
- fork (which copies ptes)

> Setting/clearing the A bit could be done via the normal set_pte pv_op,
> so that's not a big deal.
>
> Do you need to set the A bit synchronously?  

Yes, of course (if no guest cooperation).

> What happens if you install
> the guest and shadow pte with A clear, and then lazily transfer the A
> bit state from the shadow to guest pte?  Maybe at some significant event
> like  a tlb flush or:
>
>   
> > (If the host throws away a shadow page, it could sync the bits back
> > into the guest pte for safekeeping)
> >     

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.

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

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