[RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render

dongwon.kim@intel.com posted 4 patches 5 months ago
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/qemu tags/patchew/20240620231727.235841-1-dongwon.kim@intel.com
Maintainers: "Marc-André Lureau" <marcandre.lureau@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, Alex Williamson <alex.williamson@redhat.com>, "Cédric Le Goater" <clg@redhat.com>
include/hw/virtio/virtio-gpu.h  |  3 ++
include/ui/dmabuf.h             |  4 +-
include/ui/egl-helpers.h        |  3 +-
hw/display/vhost-user-gpu.c     |  3 +-
hw/display/virtio-gpu-udmabuf.c |  3 +-
hw/display/virtio-gpu.c         |  2 +
hw/vfio/display.c               |  3 +-
ui/dbus-listener.c              |  2 +-
ui/dmabuf.c                     | 11 +++-
ui/egl-helpers.c                | 27 ++++------
ui/gtk-egl.c                    | 93 ++++++++++++++++++---------------
ui/gtk-gl-area.c                | 90 +++++++++++++++++++------------
12 files changed, 146 insertions(+), 98 deletions(-)
[RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by dongwon.kim@intel.com 5 months ago
From: Dongwon Kim <dongwon.kim@intel.com>

Introducing new virtio-gpu param, 'render_sync' when guest scanout blob
is used (blob=true). The new param is used to specify when to start
rendering a guest scanout frame.

By default (and so far) rendering of the guest frame is started in
the draw event to make sure guest display update is sychronized with
host's vsync. But this method inevitably brings some extra wait because
most of time, the draw event is not happening right after the guest
scanout frame is flushed.

This delay often makes the guest stuck at certain frame for too long and
causes general performance degradation of graphic workloads on the guest's
side especially when the display update rate is high. This unwanted perf
drop can be reduced if the guest scanout frame is rendered as soon as it is
flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl display
pipeline can be unblocked a lot earlier in this case so that guest can
move to the next display frame right away.

However, this "asynchrounous" render mode may cause some waste of resources
as the guest could produce more frames than what are actually displayed
on the host display. This is similar as running rendering apps with no vblank
or vsync option. This is why this feature should stay as optional.

The param 'render_sync' is set to 'true' by default and this is in line
with traditional way while setting it to 'false' is basically enabling
this asynchronouse mode.

Dongwon Kim (4):
  hw/display/virtio-gpu: Introducing render_sync param
  ui/egl-helpers: Consolidates create-sync and create-fence
  ui/gtk-egl: Start rendering of guest blob scanout if render_sync is
    off
  ui/gtk-gl-draw: Start rendering of guest blob scanout if render_sync
    is off

 include/hw/virtio/virtio-gpu.h  |  3 ++
 include/ui/dmabuf.h             |  4 +-
 include/ui/egl-helpers.h        |  3 +-
 hw/display/vhost-user-gpu.c     |  3 +-
 hw/display/virtio-gpu-udmabuf.c |  3 +-
 hw/display/virtio-gpu.c         |  2 +
 hw/vfio/display.c               |  3 +-
 ui/dbus-listener.c              |  2 +-
 ui/dmabuf.c                     | 11 +++-
 ui/egl-helpers.c                | 27 ++++------
 ui/gtk-egl.c                    | 93 ++++++++++++++++++---------------
 ui/gtk-gl-area.c                | 90 +++++++++++++++++++------------
 12 files changed, 146 insertions(+), 98 deletions(-)

-- 
2.34.1
Re: [RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by Marc-André Lureau 4 months, 3 weeks ago
Hi

On Fri, Jun 21, 2024 at 3:20 AM <dongwon.kim@intel.com> wrote:

> From: Dongwon Kim <dongwon.kim@intel.com>
>
> Introducing new virtio-gpu param, 'render_sync' when guest scanout blob
> is used (blob=true). The new param is used to specify when to start
> rendering a guest scanout frame.
>
> By default (and so far) rendering of the guest frame is started in
> the draw event to make sure guest display update is sychronized with
> host's vsync. But this method inevitably brings some extra wait because
> most of time, the draw event is not happening right after the guest
> scanout frame is flushed.
>

> This delay often makes the guest stuck at certain frame for too long and
> causes general performance degradation of graphic workloads on the guest's
> side especially when the display update rate is high. This unwanted perf
> drop can be reduced if the guest scanout frame is rendered as soon as it is
> flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl display
> pipeline can be unblocked a lot earlier in this case so that guest can
> move to the next display frame right away.
>
> However, this "asynchrounous" render mode may cause some waste of resources
> as the guest could produce more frames than what are actually displayed
> on the host display. This is similar as running rendering apps with no
> vblank
> or vsync option. This is why this feature should stay as optional.
>

Indeed, I don't see much point in doing so, it's pure waste. If you want to
benchmark something perhaps. But then, why not simply run a benchmark
offscreen?


>
> The param 'render_sync' is set to 'true' by default and this is in line
> with traditional way while setting it to 'false' is basically enabling
> this asynchronouse mode.
>
>
As it stands now, the option should actually be on the display backend
(gtk/gtk-egl), it's not implemented for other backends.

I am not convinced this is generally useful to be an extra option though.


> Dongwon Kim (4):
>   hw/display/virtio-gpu: Introducing render_sync param
>   ui/egl-helpers: Consolidates create-sync and create-fence
>   ui/gtk-egl: Start rendering of guest blob scanout if render_sync is
>     off
>   ui/gtk-gl-draw: Start rendering of guest blob scanout if render_sync
>     is off
>
>  include/hw/virtio/virtio-gpu.h  |  3 ++
>  include/ui/dmabuf.h             |  4 +-
>  include/ui/egl-helpers.h        |  3 +-
>  hw/display/vhost-user-gpu.c     |  3 +-
>  hw/display/virtio-gpu-udmabuf.c |  3 +-
>  hw/display/virtio-gpu.c         |  2 +
>  hw/vfio/display.c               |  3 +-
>  ui/dbus-listener.c              |  2 +-
>  ui/dmabuf.c                     | 11 +++-
>  ui/egl-helpers.c                | 27 ++++------
>  ui/gtk-egl.c                    | 93 ++++++++++++++++++---------------
>  ui/gtk-gl-area.c                | 90 +++++++++++++++++++------------
>  12 files changed, 146 insertions(+), 98 deletions(-)
>
> --
> 2.34.1
>
>
>

-- 
Marc-André Lureau
Re: [RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by Kim, Dongwon 4 months, 3 weeks ago
Hi Marc-André,

On 7/2/2024 3:29 AM, Marc-André Lureau wrote:
> Hi
> 
> On Fri, Jun 21, 2024 at 3:20 AM <dongwon.kim@intel.com 
> <mailto:dongwon.kim@intel.com>> wrote:
> 
>     From: Dongwon Kim <dongwon.kim@intel.com <mailto:dongwon.kim@intel.com>>
> 
>     Introducing new virtio-gpu param, 'render_sync' when guest scanout blob
>     is used (blob=true). The new param is used to specify when to start
>     rendering a guest scanout frame.
> 
>     By default (and so far) rendering of the guest frame is started in
>     the draw event to make sure guest display update is sychronized with
>     host's vsync. But this method inevitably brings some extra wait because
>     most of time, the draw event is not happening right after the guest
>     scanout frame is flushed.
> 
> 
>     This delay often makes the guest stuck at certain frame for too long and
>     causes general performance degradation of graphic workloads on the
>     guest's
>     side especially when the display update rate is high. This unwanted perf
>     drop can be reduced if the guest scanout frame is rendered as soon
>     as it is
>     flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl display
>     pipeline can be unblocked a lot earlier in this case so that guest can
>     move to the next display frame right away.
> 
>     However, this "asynchrounous" render mode may cause some waste of
>     resources
>     as the guest could produce more frames than what are actually displayed
>     on the host display. This is similar as running rendering apps with
>     no vblank
>     or vsync option. This is why this feature should stay as optional.
> 
> 
> Indeed, I don't see much point in doing so, it's pure waste. If you want 
> to benchmark something perhaps. But then, why not simply run a benchmark 
> offscreen?

Benchmark score is not the primary reason. The problem we want to avoid 
is the laggy display update due to too many asynchronous plane updates 
happening in the guest in certain situations like when moving SW mouse 
cursor rigorously on the guest. This is because the fence (as well as 
response for the resource-flush cmd) is signaled in the draw event.

> 
> 
>     The param 'render_sync' is set to 'true' by default and this is in line
>     with traditional way while setting it to 'false' is basically enabling
>     this asynchronouse mode.
> 
> 
> As it stands now, the option should actually be on the display backend 
> (gtk/gtk-egl), it's not implemented for other backends.

We can help to deploy this concept to other backends if needed.

> 
> I am not convinced this is generally useful to be an extra option though.

I initially thought this should be the default because the virtio-gpu 
guest should not be hold by the host for too long in any cases. And this 
new approach is comparable to the default mode with blob=false where the 
guest is released as soon as the resource flush is done. Only difference 
is the resource synchronization using the fence.

> 
>     Dongwon Kim (4):
>        hw/display/virtio-gpu: Introducing render_sync param
>        ui/egl-helpers: Consolidates create-sync and create-fence
>        ui/gtk-egl: Start rendering of guest blob scanout if render_sync is
>          off
>        ui/gtk-gl-draw: Start rendering of guest blob scanout if render_sync
>          is off
> 
>       include/hw/virtio/virtio-gpu.h  |  3 ++
>       include/ui/dmabuf.h             |  4 +-
>       include/ui/egl-helpers.h        |  3 +-
>       hw/display/vhost-user-gpu.c     |  3 +-
>       hw/display/virtio-gpu-udmabuf.c |  3 +-
>       hw/display/virtio-gpu.c         |  2 +
>       hw/vfio/display.c               |  3 +-
>       ui/dbus-listener.c              |  2 +-
>       ui/dmabuf.c                     | 11 +++-
>       ui/egl-helpers.c                | 27 ++++------
>       ui/gtk-egl.c                    | 93 ++++++++++++++++++---------------
>       ui/gtk-gl-area.c                | 90 +++++++++++++++++++------------
>       12 files changed, 146 insertions(+), 98 deletions(-)
> 
>     -- 
>     2.34.1
> 
> 
> 
> 
> -- 
> Marc-André Lureau


Re: [RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by Marc-André Lureau 4 months, 3 weeks ago
Hi

On Tue, Jul 2, 2024 at 10:11 PM Kim, Dongwon <dongwon.kim@intel.com> wrote:

> Hi Marc-André,
>
> On 7/2/2024 3:29 AM, Marc-André Lureau wrote:
> > Hi
> >
> > On Fri, Jun 21, 2024 at 3:20 AM <dongwon.kim@intel.com
> > <mailto:dongwon.kim@intel.com>> wrote:
> >
> >     From: Dongwon Kim <dongwon.kim@intel.com <mailto:
> dongwon.kim@intel.com>>
> >
> >     Introducing new virtio-gpu param, 'render_sync' when guest scanout
> blob
> >     is used (blob=true). The new param is used to specify when to start
> >     rendering a guest scanout frame.
> >
> >     By default (and so far) rendering of the guest frame is started in
> >     the draw event to make sure guest display update is sychronized with
> >     host's vsync. But this method inevitably brings some extra wait
> because
> >     most of time, the draw event is not happening right after the guest
> >     scanout frame is flushed.
> >
> >
> >     This delay often makes the guest stuck at certain frame for too long
> and
> >     causes general performance degradation of graphic workloads on the
> >     guest's
> >     side especially when the display update rate is high. This unwanted
> perf
> >     drop can be reduced if the guest scanout frame is rendered as soon
> >     as it is
> >     flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl display
> >     pipeline can be unblocked a lot earlier in this case so that guest
> can
> >     move to the next display frame right away.
> >
> >     However, this "asynchrounous" render mode may cause some waste of
> >     resources
> >     as the guest could produce more frames than what are actually
> displayed
> >     on the host display. This is similar as running rendering apps with
> >     no vblank
> >     or vsync option. This is why this feature should stay as optional.
> >
> >
> > Indeed, I don't see much point in doing so, it's pure waste. If you want
> > to benchmark something perhaps. But then, why not simply run a benchmark
> > offscreen?
>
> Benchmark score is not the primary reason. The problem we want to avoid
> is the laggy display update due to too many asynchronous plane updates
> happening in the guest in certain situations like when moving SW mouse
> cursor rigorously on the guest. This is because the fence (as well as
> response for the resource-flush cmd) is signaled in the draw event.
>
>
Presumably, you won't get more frames displayed (perhaps even less due to
extra load), so why is it laggy? Is it because the guest is doing too much
buffering? Because the mouse event queue isn't being drained between
scanouts?


> >
> >
> >     The param 'render_sync' is set to 'true' by default and this is in
> line
> >     with traditional way while setting it to 'false' is basically
> enabling
> >     this asynchronouse mode.
> >
> >
> > As it stands now, the option should actually be on the display backend
> > (gtk/gtk-egl), it's not implemented for other backends.
>
> We can help to deploy this concept to other backends if needed.
>

Why not make it a gtk display option instead?


> >
> > I am not convinced this is generally useful to be an extra option though.
>
> I initially thought this should be the default because the virtio-gpu
> guest should not be hold by the host for too long in any cases. And this
> new approach is comparable to the default mode with blob=false where the
> guest is released as soon as the resource flush is done. Only difference
> is the resource synchronization using the fence.
>

virgl should be blocking rendering too

Could you detail your testing setup ?

thanks


> >
> >     Dongwon Kim (4):
> >        hw/display/virtio-gpu: Introducing render_sync param
> >        ui/egl-helpers: Consolidates create-sync and create-fence
> >        ui/gtk-egl: Start rendering of guest blob scanout if render_sync
> is
> >          off
> >        ui/gtk-gl-draw: Start rendering of guest blob scanout if
> render_sync
> >          is off
> >
> >       include/hw/virtio/virtio-gpu.h  |  3 ++
> >       include/ui/dmabuf.h             |  4 +-
> >       include/ui/egl-helpers.h        |  3 +-
> >       hw/display/vhost-user-gpu.c     |  3 +-
> >       hw/display/virtio-gpu-udmabuf.c |  3 +-
> >       hw/display/virtio-gpu.c         |  2 +
> >       hw/vfio/display.c               |  3 +-
> >       ui/dbus-listener.c              |  2 +-
> >       ui/dmabuf.c                     | 11 +++-
> >       ui/egl-helpers.c                | 27 ++++------
> >       ui/gtk-egl.c                    | 93
> ++++++++++++++++++---------------
> >       ui/gtk-gl-area.c                | 90
> +++++++++++++++++++------------
> >       12 files changed, 146 insertions(+), 98 deletions(-)
> >
> >     --
> >     2.34.1
> >
> >
> >
> >
> > --
> > Marc-André Lureau
>
>

-- 
Marc-André Lureau
Re: [RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by Kim, Dongwon 4 months, 2 weeks ago
On 7/2/2024 11:26 PM, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Jul 2, 2024 at 10:11 PM Kim, Dongwon <dongwon.kim@intel.com 
> <mailto:dongwon.kim@intel.com>> wrote:
> 
>     Hi Marc-André,
> 
>     On 7/2/2024 3:29 AM, Marc-André Lureau wrote:
>      > Hi
>      >
>      > On Fri, Jun 21, 2024 at 3:20 AM <dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com>
>      > <mailto:dongwon.kim@intel.com <mailto:dongwon.kim@intel.com>>> wrote:
>      >
>      >     From: Dongwon Kim <dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com> <mailto:dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com>>>
>      >
>      >     Introducing new virtio-gpu param, 'render_sync' when guest
>     scanout blob
>      >     is used (blob=true). The new param is used to specify when to
>     start
>      >     rendering a guest scanout frame.
>      >
>      >     By default (and so far) rendering of the guest frame is
>     started in
>      >     the draw event to make sure guest display update is
>     sychronized with
>      >     host's vsync. But this method inevitably brings some extra
>     wait because
>      >     most of time, the draw event is not happening right after the
>     guest
>      >     scanout frame is flushed.
>      >
>      >
>      >     This delay often makes the guest stuck at certain frame for
>     too long and
>      >     causes general performance degradation of graphic workloads
>     on the
>      >     guest's
>      >     side especially when the display update rate is high. This
>     unwanted perf
>      >     drop can be reduced if the guest scanout frame is rendered as
>     soon
>      >     as it is
>      >     flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl
>     display
>      >     pipeline can be unblocked a lot earlier in this case so that
>     guest can
>      >     move to the next display frame right away.
>      >
>      >     However, this "asynchrounous" render mode may cause some waste of
>      >     resources
>      >     as the guest could produce more frames than what are actually
>     displayed
>      >     on the host display. This is similar as running rendering
>     apps with
>      >     no vblank
>      >     or vsync option. This is why this feature should stay as
>     optional.
>      >
>      >
>      > Indeed, I don't see much point in doing so, it's pure waste. If
>     you want
>      > to benchmark something perhaps. But then, why not simply run a
>     benchmark
>      > offscreen?
> 
>     Benchmark score is not the primary reason. The problem we want to avoid
>     is the laggy display update due to too many asynchronous plane updates
>     happening in the guest in certain situations like when moving SW mouse
>     cursor rigorously on the guest. This is because the fence (as well as
>     response for the resource-flush cmd) is signaled in the draw event.
> 
> 
> Presumably, you won't get more frames displayed (perhaps even less due 
> to extra load), so why is it laggy? Is it because the guest is doing too 
> much buffering? 

Yes, I believe so. Virtio-gpu driver can't issue another resource flush 
as the previous frame is still on hold by the host. But there are many 
plane-update requests due to frame buffer updates because of the 
movement of SW mouse cursor in between. BTW, we currently use dirtyFB 
update on the guest running Xorg. Maybe using pageflip would make things 
better but we haven't tried it yet.

Because the mouse event queue isn't being drained
> between scanouts?



> 
>      >
>      >
>      >     The param 'render_sync' is set to 'true' by default and this
>     is in line
>      >     with traditional way while setting it to 'false' is basically
>     enabling
>      >     this asynchronouse mode.
>      >
>      >
>      > As it stands now, the option should actually be on the display
>     backend
>      > (gtk/gtk-egl), it's not implemented for other backends.
> 
>     We can help to deploy this concept to other backends if needed.
> 
> 
> Why not make it a gtk display option instead?

That is possible but I made it virtio-gpu option as this is specific to 
virtio-gpu backend. We can consider moving it to gtk layer if it makes 
more sense.

> 
> 
>      >
>      > I am not convinced this is generally useful to be an extra option
>     though.
> 
>     I initially thought this should be the default because the virtio-gpu
>     guest should not be hold by the host for too long in any cases. And
>     this
>     new approach is comparable to the default mode with blob=false where
>     the
>     guest is released as soon as the resource flush is done. Only
>     difference
>     is the resource synchronization using the fence.
> 
> 
> virgl should be blocking rendering too
> 
> Could you detail your testing setup ?

We use ubuntu linux as a guest OS but it's modified for our GFX SRIOV
setup. I am not sure if this is the details you are looking for but here 
is it: we virtualize our GPU using SRIOV. So individual linux guest have 
their dedicated portion of GPU device that is detected and worked as 
"render-only" device. And since it is render-only, we pair it with 
virtio-gpu device/driver that provides display function. Scanouts on the 
guest are allocated by virtio-gpu then imported by Mesa driver then used 
as a main framebuffer. On every plane update done by compositor will get
virtio-gpu driver to issue resource-flush (followed by set scanout) to 
host. Host (QEMU) gets this scanout data as blob (scatter-gather list) 
then creates its own dmabuf using udmabuf driver. At every resource 
flush, we block the render pipeline as well as the guest until the 
rendering if finished for scanout synchronization as you know.

We do not use virgl (instead, the normal HW driver for SRIOV-Virtual 
device is used).

> 
> thanks
> 
> 
>      >
>      >     Dongwon Kim (4):
>      >        hw/display/virtio-gpu: Introducing render_sync param
>      >        ui/egl-helpers: Consolidates create-sync and create-fence
>      >        ui/gtk-egl: Start rendering of guest blob scanout if
>     render_sync is
>      >          off
>      >        ui/gtk-gl-draw: Start rendering of guest blob scanout if
>     render_sync
>      >          is off
>      >
>      >       include/hw/virtio/virtio-gpu.h  |  3 ++
>      >       include/ui/dmabuf.h             |  4 +-
>      >       include/ui/egl-helpers.h        |  3 +-
>      >       hw/display/vhost-user-gpu.c     |  3 +-
>      >       hw/display/virtio-gpu-udmabuf.c |  3 +-
>      >       hw/display/virtio-gpu.c         |  2 +
>      >       hw/vfio/display.c               |  3 +-
>      >       ui/dbus-listener.c              |  2 +-
>      >       ui/dmabuf.c                     | 11 +++-
>      >       ui/egl-helpers.c                | 27 ++++------
>      >       ui/gtk-egl.c                    | 93
>     ++++++++++++++++++---------------
>      >       ui/gtk-gl-area.c                | 90
>     +++++++++++++++++++------------
>      >       12 files changed, 146 insertions(+), 98 deletions(-)
>      >
>      >     --
>      >     2.34.1
>      >
>      >
>      >
>      >
>      > --
>      > Marc-André Lureau
> 
> 
> 
> -- 
> Marc-André Lureau


Re: [RFC PATCH 0/4] hw/display/virtio-gpu: Introducing asynchronous guest display render
Posted by Kim, Dongwon 4 months, 3 weeks ago
On 7/2/2024 11:26 PM, Marc-André Lureau wrote:
> Hi
> 
> On Tue, Jul 2, 2024 at 10:11 PM Kim, Dongwon <dongwon.kim@intel.com 
> <mailto:dongwon.kim@intel.com>> wrote:
> 
>     Hi Marc-André,
> 
>     On 7/2/2024 3:29 AM, Marc-André Lureau wrote:
>      > Hi
>      >
>      > On Fri, Jun 21, 2024 at 3:20 AM <dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com>
>      > <mailto:dongwon.kim@intel.com <mailto:dongwon.kim@intel.com>>> wrote:
>      >
>      >     From: Dongwon Kim <dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com> <mailto:dongwon.kim@intel.com
>     <mailto:dongwon.kim@intel.com>>>
>      >
>      >     Introducing new virtio-gpu param, 'render_sync' when guest
>     scanout blob
>      >     is used (blob=true). The new param is used to specify when to
>     start
>      >     rendering a guest scanout frame.
>      >
>      >     By default (and so far) rendering of the guest frame is
>     started in
>      >     the draw event to make sure guest display update is
>     sychronized with
>      >     host's vsync. But this method inevitably brings some extra
>     wait because
>      >     most of time, the draw event is not happening right after the
>     guest
>      >     scanout frame is flushed.
>      >
>      >
>      >     This delay often makes the guest stuck at certain frame for
>     too long and
>      >     causes general performance degradation of graphic workloads
>     on the
>      >     guest's
>      >     side especially when the display update rate is high. This
>     unwanted perf
>      >     drop can be reduced if the guest scanout frame is rendered as
>     soon
>      >     as it is
>      >     flushed through 'VIRTIO_GPU_CMD_RESOURCE_FLUSH' msg. The gl
>     display
>      >     pipeline can be unblocked a lot earlier in this case so that
>     guest can
>      >     move to the next display frame right away.
>      >
>      >     However, this "asynchrounous" render mode may cause some waste of
>      >     resources
>      >     as the guest could produce more frames than what are actually
>     displayed
>      >     on the host display. This is similar as running rendering
>     apps with
>      >     no vblank
>      >     or vsync option. This is why this feature should stay as
>     optional.
>      >
>      >
>      > Indeed, I don't see much point in doing so, it's pure waste. If
>     you want
>      > to benchmark something perhaps. But then, why not simply run a
>     benchmark
>      > offscreen?
> 
>     Benchmark score is not the primary reason. The problem we want to avoid
>     is the laggy display update due to too many asynchronous plane updates
>     happening in the guest in certain situations like when moving SW mouse
>     cursor rigorously on the guest. This is because the fence (as well as
>     response for the resource-flush cmd) is signaled in the draw event.
> 
> 
> Presumably, you won't get more frames displayed (perhaps even less due 
> to extra load), so why is it laggy? Is it because the guest is doing too 
> much buffering? 

Yes, I believe so. Virtio-gpu driver can't issue another resource flush 
as the previous frame is still on hold by the host. But there are many 
plane-update requests due to frame buffer updates because of the 
movement of SW mouse cursor in between. BTW, we currently use dirtyFB 
update on the guest running Xorg. Maybe using pageflip would make things 
better but we haven't tried it yet.

Because the mouse event queue isn't being drained
> between scanouts?



> 
>      >
>      >
>      >     The param 'render_sync' is set to 'true' by default and this
>     is in line
>      >     with traditional way while setting it to 'false' is basically
>     enabling
>      >     this asynchronouse mode.
>      >
>      >
>      > As it stands now, the option should actually be on the display
>     backend
>      > (gtk/gtk-egl), it's not implemented for other backends.
> 
>     We can help to deploy this concept to other backends if needed.
> 
> 
> Why not make it a gtk display option instead?

That is possible but I made it virtio-gpu option as this is specific to 
virtio-gpu backend. We can consider moving it to gtk layer if it makes 
more sense.

> 
> 
>      >
>      > I am not convinced this is generally useful to be an extra option
>     though.
> 
>     I initially thought this should be the default because the virtio-gpu
>     guest should not be hold by the host for too long in any cases. And
>     this
>     new approach is comparable to the default mode with blob=false where
>     the
>     guest is released as soon as the resource flush is done. Only
>     difference
>     is the resource synchronization using the fence.
> 
> 
> virgl should be blocking rendering too
> 
> Could you detail your testing setup ?

We use ubuntu linux as a guest OS but it's modified for our GFX SRIOV
setup. I am not sure if this is the details you are looking for but here 
is it: we virtualize our GPU using SRIOV. So individual linux guest have 
their dedicated portion of GPU device that is detected and worked as 
"render-only" device. And since it is render-only, we pair it with 
virtio-gpu device/driver that provides display feature. Scanouts on the 
guest are allocated by virtio-gpu then imported by Mesa driver then used 
as a main framebuffer. On every plane update done by compositor will get
virtio-gpu driver to issue resource-flush (followed by set scanout) to 
host. Host (QEMU) gets this scanout data as blob (scatter-gather list) 
then creates its own dmabuf using udmabuf driver. At every resource 
flush, we block the render pipeline as well as the guest until the 
rendering if finished for scanout synchronization as you know.

We do not use virgl (instead, the normal HW driver for SRIOV-Virtual 
device is used).

So this idea of having render_sync=false is based on

1. glBlitFrameBuffer can be done anytime after resource-flush is received.
2. the guest can be unblocked whenever the blob is used (cloned by 
glBlitFrameBuffer).

> 
> thanks
> 
> 
>      >
>      >     Dongwon Kim (4):
>      >        hw/display/virtio-gpu: Introducing render_sync param
>      >        ui/egl-helpers: Consolidates create-sync and create-fence
>      >        ui/gtk-egl: Start rendering of guest blob scanout if
>     render_sync is
>      >          off
>      >        ui/gtk-gl-draw: Start rendering of guest blob scanout if
>     render_sync
>      >          is off
>      >
>      >       include/hw/virtio/virtio-gpu.h  |  3 ++
>      >       include/ui/dmabuf.h             |  4 +-
>      >       include/ui/egl-helpers.h        |  3 +-
>      >       hw/display/vhost-user-gpu.c     |  3 +-
>      >       hw/display/virtio-gpu-udmabuf.c |  3 +-
>      >       hw/display/virtio-gpu.c         |  2 +
>      >       hw/vfio/display.c               |  3 +-
>      >       ui/dbus-listener.c              |  2 +-
>      >       ui/dmabuf.c                     | 11 +++-
>      >       ui/egl-helpers.c                | 27 ++++------
>      >       ui/gtk-egl.c                    | 93
>     ++++++++++++++++++---------------
>      >       ui/gtk-gl-area.c                | 90
>     +++++++++++++++++++------------
>      >       12 files changed, 146 insertions(+), 98 deletions(-)
>      >
>      >     --
>      >     2.34.1
>      >
>      >
>      >
>      >
>      > --
>      > Marc-André Lureau
> 
> 
> 
> -- 
> Marc-André Lureau