Re: [Qemu-devel] [RFC 5/7] qxl-render: call ppm_save on callback

[Alon Levy]

On Mon, Feb 20, 2012 at 12:32:44PM +0100, Gerd Hoffmann wrote:
> On 02/19/12 22:28, Alon Levy wrote:
> > This changes the behavior of the monitor command. After the previous
> > patch, there is no longer an option of deadlock with virt-manager, but
> > ppm_save is called too early, before the update has completed. With this
> > patch it is called at the correct moment, but that means there is a race
> > between the monitor command completing and the screendump file being saved.
> > 
> > The only solution is to use an asynchronous monitor command. For a
> > previous round of this see:
> >  http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg02810.html
> > 
> > Since that's contentious, I'm suggesting we do something that is almost
> > correct and doesn't hang, instead of correct and hangs. The screendump
> > user can inotify on the directory and the file if need be. For casual
> > monitor usage there is no difference.
> 
> Hmm, that is pretty lame.  There are users like autotest which expect
> the screen dump being there when the monitor command is finished, that
> change will break them.
> 
> Unfortunaly there is no easy way out.  I think the options are:
> 
>  (1) Keep existing behavior.  That means the screenshot might show old
>      screen content.  Not very nice too.  Would work sort-of ok for
>      autotest though as autotest does screenshots every second and thus
>      the screen content wouldn't be older than a second.
> 
>  (2) Async monitor command.  Keeps interface and works nicely.  A bunch
>      of QAPI bits tickled into master meanwhile, so we could look at
>      this again.  Luiz?  What is the status here?
> 
>  (3) Something like this patch + additionally introduce a
>      "your-screenshot-is-finished-now" qmp event.  Will break existing
>      users too.  But at least they can be adapted without requiring
>      some external, nonportable service like inotify ...
> 

I was going to look for QAPI bits after this series (i.e. (2)). Doing
(3) is also possible.

> cheers,
>   Gerd
>