Re: [RFC PATCH 6/6] drm/i915/gvt: support QEMU getting the dmabuf

[Gerd Hoffmann]

  Hi,

> If the contents of the framebuffer change or if the parameters of the
> framebuffer change?  I can't image that creating a new dmabuf fd for
> every visual change within the framebuffer would be efficient, but I
> don't have any concept of what a dmabuf actually does.

Ok, some background:

The drm subsystem has the concept of planes.  The most important plane
is the primary framebuffer (i.e. what gets scanned out to the physical
display).  The cursor is a plane too, and there can be additional
overlay planes for stuff like video playback.

Typically there are multiple planes in a system and only one of them
gets scanned out to the crtc, i.e. the fbdev emulation creates one plane
for the framebuffer console.  The X-Server creates a plane too, and when
you switch between X-Server and framebuffer console via ctrl-alt-fn the
intel driver just reprograms the encoder to scan out the one or the
other plane to the crtc.

The dma-buf handed out by gvt is a reference to a plane.  I think on the
host side gvt can see only the active plane (from encoder/crtc register
programming) not the inactive ones.

The dma-buf can be imported as opengl texture and then be used to render
the guest display to a host window.  I think it is even possible to use
the dma-buf as plane in the host drm driver and scan it out directly to
a physical display.  The actual framebuffer content stays in gpu memory
all the time, the cpu never has to touch it.

It is possible to cache the dma-buf handles, i.e. when the guest boots
you'll get the first for the fbcon plane, when the x-server starts the
second for the x-server framebuffer, and when the user switches to the
text console via ctrl-alt-fn you can re-use the fbcon dma-buf you
already have.

The caching becomes more important for good performance when the guest
uses pageflipping (wayland does): define two planes, render into one
while displaying the other, then flip the two for a atomic display
update.

The caching also makes it a bit difficult to create a good interface.
So, the current patch set creates:

  (a) A way to query the active planes (ioctl
      INTEL_VGPU_QUERY_DMABUF added by patch 5/6 of this series).
  (b) A way to create a dma-buf for the active plane (ioctl
      INTEL_VGPU_GENERATE_DMABUF).

Typical userspace workflow is to first query the plane, then check if it
already has a dma-buf for it, and if not create one.

> What changes to the framebuffer require a new dmabuf fd?  Shouldn't the
> user query the parameters of the framebuffer through a dmabuf fd and
> shouldn't the dmabuf fd have some signaling mechanism to the user
> (eventfd perhaps) to notify the user to re-evaluate the parameters?

dma-bufs don't support that, they are really just a handle to a piece of
memory, all metadata (format, size) most be communicated by other means.

> Otherwise are you imagining that the user polls the vfio region?

Hmm, notification support would probably a good reason to have a
separate file handle to manage the dma-bufs (instead of using
driver-specific ioctls on the vfio fd), because the driver could also
use the management fd for notifications then.

I'm not sure how useful notification support actually is though.
Notifications when another plane gets mapped to the crtc should be easy.
But I'm not sure it is possible to get notifications when the plane
content changes, especially in case the guest does software rendering so
the display is updated without gvt seeing guest activity on the
rendering pipeline.  Without the later qemu needs a timer for display
updates _anyway_ ...

cheers,
  Gerd