[Spice-devel] UMS memory management

[Hans de Goede]

Hi,

On 03/27/2013 09:07 AM, Søren Sandmann wrote:
> Hello,
>
> The following is some notes on what I am planning to do for UMS memory
> management. It basically amounts to a rewrite of qxl-surface-ums.c

Looks / sounds good, a few remarks below (note I'm not an expert on this piece
of the spice code).

<snip>

>       "in host memory":
>          - no associated ID
>          - has associated pixmap
>          - pixman image may not exist

Hmm, I assume with host-memory you mean guest memory here, right? iow surfaces
will be moved by the driver from video-memory to X-server allocated memory (which
in a vm is guest memory). I'm assuming this is what you mean in the rest of
my reply.

<snip>


> Details:
>
> ** IN_VIDEO => IN_HOST
>
> - update_area is issued for the surface in question
> - data is copied to pixman image
> - state is set to IN_HOST
> - destroy command is issued for the ID

I assume the main use-case for this is freeing up video memory,
unfortunately due to glz compression and the drawing command tree
kept in the spice-server, it is possible that the server itself
still holds a reference to the surface, and sending the destroy
will not end up freeing any memory (atleast not directly), this
is esp. going to be a problem when doing these kind of evictions
with the intention of freeing up a larger block of memory
for a new surface.

I've been discussing this with Alon when we were in Brno together,
and I think we need a new qxl device io-command for this, where the
driver can tell it wants to video memory to be releases *now*
rather then when the server looses it last ref, and then the
server would need to do an eviction of its own, moving the surface
from (shared) video memory to some memory inside the qemu process.

Note that we will want to limit the amount of memory which can be
use spice-server-side by these evicted, to avoid this being used
as some kind of DOS attack, but this should be a limit which is
(hardly) ever reached in practice.

Implementing this won't be trivial, but if we all agree it is
something which we want to have, then I hope we can find someone
in the spice team to start working on this.

Regards,

Hans