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

[Tomeu Vizoso]

On 02/12/2018 12:45 PM, Gerd Hoffmann wrote:
>    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?

Guess it could be that, but I was rather thinking of clients that would 
allocate the buffer for wl_shm_pool with DRM_VIRTGPU_RESOURCE_CREATE or 
equivalent. Then that buffer would be exported and the fd passed using 
the standard wl_shm protocol.

> > > > 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.

Would it make sense for virtio-gpu to map buffers to the guest via PCI 
BARs? So we can use a single drm driver for both 2d and 3d.

> > > > 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.

Yeah, would be great if we could find a way to avoid that.

> 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?

I think that should be fine for now. Something similar will have to 
happen for the clipboard, which currently uses pipes to exchange data.

Thanks,

Tomeu