Re: [PATCH v3 1/2] drm/virtio: Add window server support

[Gerd Hoffmann]

  Hi,

> >    (a) software rendering: client allocates shared memory buffer, renders
> >        into it, then passes a file handle for that shmem block together
> >        with some meta data (size, format, ...) to the wayland server.
> > 
> >    (b) gpu rendering: client opens a render node, allocates a buffer,
> >        asks the cpu to renders into it, exports the buffer as dma-buf
> >        (DRM_IOCTL_PRIME_HANDLE_TO_FD), passes this to the wayland server
> >        (again including meta data of course).
> > 
> > Is that correct?
> 
> Both are correct descriptions of typical behaviors. But it isn't spec'ed
> anywhere who has to do the buffer allocation.

Well, according to Pekka's reply it is spec'ed that way, for the
existing buffer types.  So for server allocated buffers you need
(a) a wayland protocol extension and (b) support for the extension
in the clients.

> That's to say that if we cannot come up with a zero-copy solution for
> unmodified clients, we should at least support zero-copy for cooperative
> clients.

"cooperative clients" == "client which has support for the wayland
protocol extension", correct?

> > > Creation of shareable buffer by guest
> > > -------------------------------------------------
> > > 
> > > 1. Client requests virtio driver to create a buffer suitable for sharing
> > > with host (DRM_VIRTGPU_RESOURCE_CREATE)
> > 
> > client or guest proxy?
> 
> As per the above, the GUI toolkit could have been modified so the client
> directly creates a shareable buffer, and renders directly to it without any
> extra copies.
> 
> If clients cannot be modified, then it's the guest proxy what has to create
> the shareable buffer and keep it in sync with the client's non-shareable
> buffer at the right times, by intercepting wl_surface.commit messages and
> copying buffer contents.

Ok.

> > > 4. QEMU maps that buffer to the guest's address space
> > > (KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver
> > 
> > That part is problematic.  The host can't simply allocate something in
> > the physical address space, because most physical address space
> > management is done by the guest.  All pci bars are mapped by the guest
> > firmware for example (or by the guest OS in case of hotplug).
> 
> How can KVM_SET_USER_MEMORY_REGION ever be safely used then? I would have
> expected that callers of that ioctl have enough knowledge to be able to
> choose a physical address that won't conflict with the guest's kernel.

Depends on the kind of region.  Guest RAM is allocated and mapped by
qemu, guest firmware can query qemu about RAM mappings using a special
interface, then create a e820 memory map for the guest os.  PCI device
bars are mapped according to the pci config space registers, which in
turn are initialized by the guest firmware, so it is basically in the
guests hand where they show up.

> I see that the ivshmem device in QEMU registers the memory region in BAR 2
> of a PCI device instead. Would that be better in your opinion?

Yes.

> > > 4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each
> > > resource, sends data + FDs to the compositor with SCM_RIGHTS
> > 
> > BTW: Is there a 1:1 relationship between buffers and shmem blocks?  Or
> > does the wayland protocol allow for offsets in buffer meta data, so you
> > can place multiple buffers in a single shmem block?
> 
> The latter:
> https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_shm_pool

Ah, good, that makes it alot easier.

So, yes, using ivshmem would be one option.  Tricky part here is the
buffer management though.  It's just a raw piece of memory.  The guest
proxy could mmap the pci bar and manage it.  But then it is again either
unmodified guest + copying the data, or modified client (which requests
buffers from guest proxy) for zero-copy.

Another idea would be extending stdvga.  Basically qemu would have to
use shmem as backing storage for vga memory instead of anonymous memory,
so it would be very  simliar to ivshmem on the host side.  But on the
guest side we have a drm driver for it (bochs-drm).  So clients can
allocate dumb drm buffers for software rendering, and the buffer would
already be backed by a host shmem segment.  Given that wayland already
supports drm buffers for 3d rendering that could work without extending
the wayland protocol.  The client proxy would have to translate the drm
buffer into an pci bar offset and pass it to the host side.  The host
proxy could register the pci bar as wl_shm_pool, then just pass through
the offset to reference the individual buffers.

Drawback of both approaches would be that software rendering and gpu
rendering would use quite different code paths.

We also need a solution for the keymap shmem block.  I guess the keymap
doesn't change all that often, so maybe it is easiest to just copy it
over (host proxy -> guest proxy) instead of trying to map the host shmem
into the guest?

cheers,
  Gerd