Re: [Qemu-devel] [PATCH 1/5] monitor: screen_dump async

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 24 Oct 2011 19:29:37 +0200
Alon Levy <alevy@redhat.com> wrote:

> On Mon, Oct 24, 2011 at 01:45:16PM -0200, Luiz Capitulino wrote:
> > On Mon, 24 Oct 2011 17:13:14 +0200
> > Gerd Hoffmann <kraxel@redhat.com> wrote:
> > 
> > > On 10/24/11 14:02, Alon Levy wrote:
> > > > Make screen_dump monitor command an async command to allow next for qxl
> > > > to implement it as a initiating call to red_worker and completion on
> > > > callback, to fix a deadlock when issueing a screendump command via
> > > > libvirt while connected with a libvirt controlled spice-gtk client.
> > > 
> > > Approach looks reasonable to me.  Patch breaks the build though, you've
> > > missed a bunch of screen_dump functions in non-x86 targets.
> > 
> > There are two problems actually.
> > 
> > The first one is that changing an existing command from synchronous
> > to asynchronous is an incompatible change because asynchronous commands
> > semantics is different. For an example of possible problems please
> > check: https://bugzilla.redhat.com/show_bug.cgi?id=623903.
> > 
> > The second problem is that the existing asynchronous interface in the
> > monitor is incomplete and has never been used for real. Our plan is to
> > use QAPI's async support, but that has not landed in master yet and iirc
> > there wasn't consensus about it. I also think it's a bit late for its
> > inclusion in 1.0 (and certainly not a candidate for stable).
> > 
> > If all you need here is to delay sending the response, then maybe the
> > current interface could work (although I honestly don't trust it and
> > regret not having dropped it). Otherwise our only choice would be to
> > work on getting the QAPI async support merged.
> 
> My problem is that the io thread keeps the global mutex during the wait,
> that's why the async monitor is perfect for what I want - this is
> exactly what it does.

Let's not mix internal implementation details with what we want as
an external interface.

Can't you just make a vga_hw_screen_dump() specific callback?

> I haven't looked at QAPI async support, but I
> understand it's a bit in the future.

Yes, it's not for the immediate term.