Re: [Qemu-devel] [RFC v4 9/9] qxl-render: call ppm_save on bh

[Alon Levy]

On Wed, Feb 22, 2012 at 12:46:28PM +0100, Gerd Hoffmann wrote:
> On 02/21/12 22:39, 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.
> 
> I still think we should defer that and figure how to fix that properly,
> either using (internally) async monitor commands via qapi, or using an
> event.
> 
> > +typedef struct QXLPPMSaveBHData {
> > +    PCIQXLDevice *qxl;
> > +    QXLCookie *cookie;
> > +} QXLPPMSaveBHData;
> 
> Is there a need for a separate struct?  I think you can just stick the
> filename into the QXLCookie struct, then write out screen shots in the
> update area bottom half, no?

You don't get the cookie in the update_area bottom half. It's solemnly
for update_area_complete. It's not easy to associate a cookie with it,
since it may be called by the server without previous provocation from
qemu.

I'll take another look.

> 
> cheers,
>   Gerd
> 
>