Re: [Qemu-devel] [PATCH] Add new client_present and client capabilities fields to QXLRom

[sandmann@cs.au.dk (Søren Sandmann)]

Gerd Hoffmann <kraxel@redhat.com> writes:

> On 08/27/12 19:20, Søren Sandmann Pedersen wrote:
>> From: Søren Sandmann Pedersen <ssp@redhat.com>
>> 
>> The client_present field is a byte that is set of non-zero when a
>> client is connected and to zero when no client is connected.
>> 
>> The client_capabilities[58] array contains 464 bits that indicate the
>> capabilities of the client.
>
> What is supposed to happen in case multiple clients are connected?

Is this case supported at all?

If it is, I'd say that the guest should not be aware of it and the bits
advertise should be interpreted as "these are the capabilities that
spice-server will marshall on to the clients that are
connected". Presumably spice-server would then set the bit vector to the
intersection of all the clients.

> How do you handle the race conditions, especially on capability
> downgrade?  There might be not-yet processed commands in the command
> queue which the client is unable to handle, or existing surfaces using
> formats the client doesn't understand ...

Good question. 

I don't know of a good way to deal with the situation where the new
client is unable to handle existing surfaces. I suppose in principle
spice-server could emulate their existence, sending them as images, but
I'm not familiar enough with spice-server to say whether that is
feasible.

For commands, would it work for spice-server to just process everything
in the command ring after changing the capability bits (ie., in possibly
brief moment before a new client connects)? It seems that would be a
good thing to do even without capability bits.


Søren