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Date: Tue, 11 Sep 2012 10:45:22 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] hw/qxl: support client monitor
 configuration via device
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To: Alon Levy <alevy@redhat.com>
Cc: qemu-devel@nongnu.org

On 09/11/12 08:56, Alon Levy wrote:
> Until now we used only the agent to change the monitor count and each
> monitor resolution. This patch introduces the qemu part of using the
> device as the mediator instead of the agent via virtio-serial.
> 
> Spice (>=0.11.5) calls the new QXLInterface::client_monitors_config,
> generating an interrupt QXL_INTERRUPT_CLIENT_MONITORS_CONFIG which the
> client indicates handling of (after reading from
> QXLRom::client_monitors_config) by QXL_IO_CLIENT_MONITORS_CONFIG_DONE.

I don't think an explicit handshake via
QXL_IO_CLIENT_MONITORS_CONFIG_DONE is a good idea.

How about this update protocol:

qemu:
  (1) set QXLRom::client_monitors_config_updating
  (2) fill QXLRom::client_monitors_config
  (3) raise QXL_INTERRUPT_CLIENT_MONITORS_CONFIG
  (4) clear QXLRom::client_monitors_config_updating

guest:
  (1) clear QXL_INTERRUPT_CLIENT_MONITORS_CONFIG bit in irq status
  (2) wait until QXLRom::client_monitors_config_updating is clear
  (3) parse QXLRom::client_monitors_config
  (4) check QXL_INTERRUPT_CLIENT_MONITORS_CONFIG bit in irq status
      (a) when set, goto (1).
      (b) when clear we are done.

While thinking about it:  I think we also don't need the guest
capabilities.  With the handshake gone we can update
QXLRom::client_monitors_config unconditionally.

We might want to notify spice-server when the guest flips the
QXL_INTERRUPT_CLIENT_MONITORS_CONFIG bit in the irq mask, so we can
route the event accordingly.  Or we just route it unconditionally both
ways and let the guest sort it (i.e. make vdagent ignore monitors config
when the qxl kms driver is active).

cheers,
  Gerd