Re: [Xen-devel] [PATCH 12/17] xenpaging: handle HVMCOPY_gfn_paged_out in copy_from/to_user

[Keir Fraser <keir@xxxxxxx>]

On 06/12/2010 20:59, "Olaf Hering" <olaf@xxxxxxxxx> wrote:

>      mem_event_put_request(d, &req);
> +
> +    if ( req.flags & MEM_EVENT_FLAG_VCPU_PAUSED )
> +    {
> +        wait_event(d->wq, mfn_valid(gfn_to_mfn(p2m, gfn, &p2mt)) &&
> !p2m_is_paging(p2mt));
> +    }
> +

This I find interesting. Do you not race the xenpaging daemon satisfying
your page-in request, but then very quickly paging it out again? In which
case you might never wake up!

I think the condition you wait on should be for a response to your paging
request. A wake_up() alone is not really sufficient; you need some kind of
explicit flagging to the vcpu too. Could the paging daemon stick a response
in a shared ring, or otherwise explicitly flag to this vcpu that it's
request has been fully satisfied and it's time to wake up and retry its
operation? Well, really that's a rhetorical question, because that is
exactly what you need to implement for this waitqueue strategy to work
properly!

 -- Keir



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