Re: [Qemu-devel] Unknown command 0xffffff in SVGA command FIFO

[Janne Huttunen <jahuttun@gmail.com>]

> No, I think that can't happen, but it would be interesting to bisect
> what the guest is doing exactly when this happens.  The guest should
> not move the "next command" pointer before if has written the command
> entirely, this should be enough to guard against executing a partial
> command.  Unless there's another timing issue somewhere obviously.

Well, the guest driver is essentially the one in X.Org git tree here:

http://cgit.freedesktop.org/xorg/driver/xf86-video-vmware/tree/src

Looking at e.g. vmwareSendSVGACmdUpdate and vmwareWriteWordToFIFO
in vmware.c, the command seems to be inserted into the FIFO one
value at a time. Now, is the whole sequence somehow atomic wrt the
host FIFO access or not?

> Hmm, I don't know about the guest.. would be good to check too, but it
> should be only fixable in the guest if it is so.

Is the FIFO protocol documented somewhere? Like what kind of
atomicity is expected?

> I'm not sure if it's likely that the FIFO is really getting full?
> Most guest software will not write too many commands without knowing
> that the previous content has appeared on the screen, right?

Yes, I agree. That's more of a theoretical issue.

> The FIFO can run at any moment but everything else stops until all the
> commands currently in the FIFO have been executed.  vmware_fifo_run is
> called by the UI update which in turn is called from the main select()
> loop.  Guest code is also executed in that loop.

"At any time" as in between guest calls to vmwareWriteWordToFIFO?
Or not? It seems to me that the GUI is updated from a timer, but
can it go off at any time?