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

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 02/06/2018 03:23 PM, Gerd Hoffmann wrote:
>    Hi,
> 
>>> Hmm?  I'm assuming the wayland client (in the guest) talks to the
>>> wayland proxy, using the wayland protocol, like it would talk to a
>>> wayland display server.  Buffers must be passed from client to
>>> server/proxy somehow, probably using fd passing, so where is the
>>> problem?
>>>
>>> Or did I misunderstand the role of the proxy?
>>
>> Hi Gerd,
>>
>> it's starting to look to me that we're talking a bit past the other, so I
>> have pasted below a few words describing my current plan regarding the 3 key
>> scenarios that I'm addressing.
> 
> You are describing the details, but I'm missing the big picture ...
> 
> So, virtualization aside, how do buffers work in wayland?  As far I know
> it goes like this:
> 
>    (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.

In practical terms, the buffer allocation happens in either the 2D GUI 
toolkit (gtk+, for example), or the EGL implementation. Someone using 
this in a real product would most probably be interested in avoiding any 
extra copies and make sure that both allocate buffers via virtio-gpu, for 
example.

Depending on the use case, they could be also interested in supporting 
unmodified clients with an extra copy per buffer presentation.

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.

> Now, with virtualization added to the mix it becomes a bit more
> complicated.  Client and server are unmodified.  The client talks to the
> guest proxy (wayland protocol).  The guest proxy talks to the host proxy
> (protocol to be defined). The host proxy talks to the server (wayland
> protocol).
> 
> Buffers must be managed along the way, and we want avoid copying around
> the buffers.  The host proxy could be implemented directly in qemu, or
> as separate process which cooperates with qemu for buffer management.
> 
> Fine so far?

Yep.

>> I really think that whatever we come up with needs to support 3D clients as
>> well.
> 
> Lets start with 3d clients, I think these are easier.  They simply use
> virtio-gpu for 3d rendering as usual.  When they are done the rendered
> buffer already lives in a host drm buffer (because virgl runs the actual
> rendering on the host gpu).  So the client passes the dma-buf to the
> guest proxy, the guest proxy imports it to look up the resource-id,
> passes the resource-id to the host proxy, the host proxy looks up the
> drm buffer and exports it as dma-buf, then passes it to the server.
> Done, without any extra data copies.

Yep.

>> 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.

>> 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.

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?

>> 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

Regards,

Tomeu