Re: [Qemu-devel] [PATCH v2 1/2] console: add hw_screen_dump_async

[Alon Levy]

On Tue, Mar 06, 2012 at 09:10:00AM +0100, Gerd Hoffmann wrote:
> On 03/06/12 08:56, Alon Levy wrote:
> > On Tue, Mar 06, 2012 at 08:36:34AM +0100, Gerd Hoffmann wrote:
> >>   Hi,
> >>
> >>>> How would the parallel execution facility be opaque to the implementer?
> >>>> screendump returns, screendump_async needs to pass a closure. You can
> >>>> automatically generate any amount of code, but you can only have a
> >>>> single function implementation with longjmp/coroutine, or having a
> >>>> saparate thread per command but that would mean taking locks for
> >>>> anything not trivial, which avoids the no-change-to-implementation. Is
> >>>> this what you have in mind?
> >>>
> >>> It would not be opaque to the implementer.  But it would avoid
> >>> introducing new commands and events, instead we have a unified mechanism
> >>> to signal completion.
> >>
> >> Ok.  We have a async mechanism today: .mhandler.cmd_async = ...
> >>
> >> I know it has its problems like no cancelation and is deprecated and
> >> all.  But still: how about using it as interim until QAPI-based async
> >> monitor support is ready?  We could unbreak qxl screendumps without
> >> having to introduce a new (but temporary!) screendump_async command +
> >> completion event.
> > 
> > Actually, I'm not sure this doesn't reintroduce the original problem
> > (which I haven't been able to reproduce):
> > 
> > client: screenshot <-> client libvirt <-> host libvirt
> > 
> > host libvirt (screendump) <-> qemu monitor -> <- spice server <-> client
> 
> Hmm?  spice client can ask for a screendump via libvirt?
> 
> /me looks completely puzzled.

No, ok, bad attempt.

 "client" is a libvirt and spice client.
 client as a libvirt client asks for screenshot.
 libvirt (client side) asks from libvirt (host side)
 libvirt (host side) issues a screendump monitor command (the new
   internal async version)
 qxl_screen_dump asks spice server to render
 spice server waits for pipe to spice client to empty (lower then 50)

 but spice client, which is just "client", is blocking on completion of
 the screendump command.

 I have two problems with my own explanation:
  1. I didn't manage to reproduce it myself, and nor has Marc Andre (who
  first reported this problem) yesterday.
  2. I remember that the async monitor command I sent in October did fix
  the problem, so something with my description is wrong.

> 
> cheers,
>   Gerd
> 
>