Re: Populating multiple ptes at fault time

[Jeremy Fitzhardinge]

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.

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?  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)


    J
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