Re: [RFC PATCH] drm/vkms: Add support for virtual hardware mode

[Gerd Hoffmann]

On Thu, Feb 25, 2021 at 11:32:08AM +0100, Daniel Vetter wrote:
> On Thu, Feb 25, 2021 at 11:25:20AM +0100, Gerd Hoffmann wrote:
> > On Thu, Feb 25, 2021 at 10:09:42AM +0100, Daniel Vetter wrote:
> > > On Wed, Feb 24, 2021 at 11:55 AM Sumera Priyadarsini
> > > <sylphrenadin@xxxxxxxxx> wrote:
> > > >
> > > > Add a virtual hardware or vblank-less mode as a module to enable
> > > > VKMS to emulate virtual graphic drivers. This mode can be enabled
> > > > by setting enable_virtual_hw=1 at the time of loading VKMS.
> > > >
> > > > A new function vkms_crtc_composer() has been added to bypass the
> > > > vblank mode and is called directly in the atomic hook in
> > > > vkms_atomic_begin(). However, some crc captures still use vblanks
> > > > which causes the crc-based igt tests to crash. Currently, I am unsure
> > > > about how to approach one-shot implementation of crc reads so I am
> > > > still working on that.
> > > 
> > > Gerd, Zack: For virtual hw like virtio-gpu or vmwgfx that does
> > > one-shot upload and damage tracking, what do you think is the best way
> > > to capture crc for validation? Assuming that's even on the plans
> > > anywhere ...
> > > 
> > > Ideally it'd be a crc that the host side captures, so that we really
> > > have end-to-end validation, including the damage uploads and all that.
> > 
> > Disclaimer:  Not knowing much about the crc thing beside having noticed
> > it exists and seems to be used for display content checking.
> > 
> > > For vkms we're going for now with one-shot crc generation after each
> > > atomic flip (or DIRTYFB ioctl call). Will need a pile of igt changes,
> > > but seems like the most fitting model.
> > > Other option would be that we'd wire up something on the kernel side
> > > that generates a crc on-demand every time igt reads a new crc value
> > > (maybe with some rate limiting). But that's not really how virtual hw
> > > works when everything is pushed explicitly to the host side.
> > 
> > igt runs inside the guest, right?
> 
> Yup. There's some debugfs files for capture crc on a specific CRTC. So
> supporting this would mean some virtio-gpu revision so you could ask the
> host side for a crc when you do a screen update, and the host side would
> send that back to you on a virtio channel as some kind of message.

Waded through the source code a bit.  So, the vkms crc code merges all
planes (specifically the cursor plane) before calculating the crc.
Which is a bit of a problem, we try to avoid that and rarely actually
merge the planes anywhere in the virtualization stack.  Instead we
prefer to pass through the cursor plane separately, so we can -- for
example -- use that to simply set the cursor sprite of the qemu gtk
window.  It's much more snappy because moving+rendering the pointer
doesn't need a round-trip to the guest then.

So, it would be quite some effort on the host side, we would have to
merge planes just for crc calculation.

> > You can ask qemu to write out a screen dump.

Hmm, the (hardware) cursor is not in the screen dump either.

A software cursor (when using for example cirrus which has no cursor
plane) would be there.

> > Another option to access the screen would be vnc.

vnc clients can get the cursor sprite.  They can't get the position
though, only set it (it's a one-way ticket in the vnc protocol).
Typically not a problem because desktops set the position in response
to the pointer events so guest + host position match nevertheless.
But for test cases which don't look at input events and set the cursor
to a fixed place this is a problem ...

> > On-demand crc via debugfs or ioctl would work too, but yes that wouldn't
> > verify the host-side.  At least not without virtio protocol extensions.
> > We could add a new command asking the host to crc the display and return
> > the result for on-demand crc.  Or add a crc request flag to an existing
> > command (VIRTIO_GPU_CMD_RESOURCE_FLUSH probably).
> 
> Yup, I think that's what would be needed. The question here is, what do
> you think would be the most natural way for virtio host side stack to
> support this?

virtio has feature flags, so we can easily introduce an extension in a
backward compatible way.  Each command sends a reply, with optional
payload, so it would make sense to send the crc with the
VIRTIO_GPU_CMD_RESOURCE_FLUSH reply.

Alternatively introduce a communication channel independent of the gpu,
using for example virtio-serial or vsock, let the guest send crc
requests to qemu that way.  Which would work with all qemu display
devices, not only virtio-gpu.

take care,
  Gerd