Date:   Tue, 6 Feb 2018 13:41:52 +0100
From:   Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Zach Reizner <zachr@...gle.com>,
        kernel@...labora.com, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Jason Wang <jasowang@...hat.com>,
        Stefan Hajnoczi <stefanha@...il.com>
Subject: Re: [PATCH v3 1/2] drm/virtio: Add window server support

On 02/05/2018 05:03 PM, Gerd Hoffmann wrote:
> On Mon, Feb 05, 2018 at 03:46:17PM +0100, Tomeu Vizoso wrote:
>> On 02/05/2018 01:20 PM, Gerd Hoffmann wrote:
>>>     Hi,
>>>
>>>>> Why not use virtio-vsock to run the wayland protocol? I don't like
>>>>> the idea to duplicate something with very simliar functionality in
>>>>> virtio-gpu.
>>>>
>>>> The reason for abandoning that approach was the type of objects that
>>>> could be shared via virtio-vsock would be extremely limited. Besides
>>>> that being potentially confusing to users, it would mean from the
>>>> implementation side that either virtio-vsock would gain a dependency on
>>>> the drm subsystem, or an appropriate abstraction for shareable buffers
>>>> would need to be added for little gain.
>>>
>>> Well, no.  The idea is that virtio-vsock and virtio-gpu are used largely
>>> as-is, without knowing about each other.  The guest wayland proxy which
>>> does the buffer management talks to both devices.
>>
>> Note that the proxy won't know anything about buffers if clients opt-in for
>> zero-copy support (they allocate the buffers in a way that allows for
>> sharing with the host).
> 
> 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.

I mention below KVM_SET_USER_MEMORY_REGION, but I guess we can discuss 
alternatives such as the one you are proposing using PCI BARs at a later 
stage.

I really think that whatever we come up with needs to support 3D clients 
as well.


Creation of shareable buffer by guest
-------------------------------------------------

1. Client requests virtio driver to create a buffer suitable for sharing 
with host (DRM_VIRTGPU_RESOURCE_CREATE)

2. Virtio driver creates a new resource ID and passes the request to 
QEMU (VIRTIO_GPU_CMD_RESOURCE_CREATE_2D)

3. QEMU creates a shmem file (for example with mkostemp), associates 
that FD with the ID of this resource

4. QEMU maps that buffer to the guest's address space 
(KVM_SET_USER_MEMORY_REGION), passes the guest PFN to the virtio driver

5. DRM_VIRTGPU_RESOURCE_CREATE returns the resource id just created

6. Client mmaps it with DRM_IOCTL_VIRTGPU_MAP+mmap and can render to it

7. Gets a FD with DRM_IOCTL_PRIME_HANDLE_TO_FD that can be sent around


Send of shareable buffer by guest
---------------------------------------------

1. Client sends the host a message that refers to this buffer, passing 
the FD using SCM_RIGHTS

2. Guest proxy passes the message (serialized data + FDs) on to the 
virtio driver responsable for winsrv support

3. virtio driver puts the data and the resource ids corresponding to the 
FDs in a virtqueue, kicks it

4. QEMU pops data+buffers from the virtqueue, looks up shmem FD for each 
resource, sends data + FDs to the compositor with SCM_RIGHTS


Reception of buffer from the compositor
-----------------------------------------------------

1. QEMU reads from the socket and gets a FD via SCM_RIGHTS

2. QEMU mmaps the FD and maps the resulting pointer to the guest via 
KVM_SET_USER_MEMORY_REGION

3. QEMU sends the guest PFN along the presentation data to the virtio 
driver (VIRTIO_GPU_CMD_WINSRV_RX)

4. Virtio driver wraps a FD around that PFN, puts it in a queue

5. Guest proxy calls DRM_IOCTL_VIRTGPU_WINSRV_RX and gets data plus that FD

6. Guest proxy sends that data + FD to the client via SCM_RIGHTS

7. Client gets FD, mmaps it and reads the data from the compositor



Thanks,

Tomeu


>>>>> If you have a guest proxy anyway using virtio-sock for the protocol
>>>>> stream and virtio-gpu for buffer sharing (and some day 3d rendering
>>>>> too) should work fine I think.
>>>>
>>>> If I understand correctly your proposal, virtio-gpu would be used for
>>>> creating buffers that could be shared across domains, but something
>>>> equivalent to SCM_RIGHTS would still be needed in virtio-vsock?
>>>
>>> Yes, the proxy would send a reference to the buffer over virtio-vsock.
>>> I was more thinking about a struct specifying something like
>>> "ressource-id 42 on virtio-gpu-pci device in slot 1:23.0" instead of
>>> using SCM_RIGHTS.
>>
>> Can you extend on this? I'm having trouble figuring out how this could work
>> in a way that keeps protocol data together with the resources it refers to.
> 
> Don't know much about the wayland protocol.  Assuming you are passing
> buffers as file handles, together with some information what kind of
> buffer this is (sysv shm, dma-buf, ...).
> 
> We have a proxy on both ends.  One running in the guest, one on the host
> (be it qemu or some external one).  So these two have to agree on how to
> pass buffers from one to the other.  One way would be to have them talk
> a simple meta protocol to each other, with "here comes a chunk wayland
> protocol to pass along" and "here is a buffer mgmt message".  Possibly
> it is better to extend the wayland protocol to also cover this new kind
> of buffer, so you don't need the meta protocol.
> 
> The proxies would talk normal wayland protocol to the client (in the
> guest) and the server (on the host).  They will have to transform the
> buffer into something they can pass along using the wayland protocol.
> 
>>>>> What is the plan for the host side? I see basically two options. Either
>>>>> implement the host wayland proxy directly in qemu. Or
>>>>> implement it as separate process, which then needs some help from
>>>>> qemu to get access to the buffers. The later would allow qemu running
>>>>> independant from the desktop session.
>>>>
>>>> Regarding synchronizing buffers, this will stop becoming needed in
>>>> subsequent commits as all shared memory is allocated in the host and
>>>> mapped to the guest via KVM_SET_USER_MEMORY_REGION.
>>>
>>> --verbose please.  The qemu patches linked from the cover letter not
>>> exactly helpful in understanding how all this is supposed to work.
>>
>> A client will allocate a buffer with DRM_VIRTGPU_RESOURCE_CREATE, export it
>> and pass the FD to the compositor (via the proxy).
>>
>> During resource creation, QEMU would allocate a shmem buffer and map it into
>> the guest with KVM_SET_USER_MEMORY_REGION.
> 
> So the buffer magically shows up somewhere in the physical address space
> of the guest?  That kind if magic usually isn't a very good idea.
> 
>> When a FD comes from the compositor, QEMU mmaps it and maps that virtual
>> address to the guest via KVM_SET_USER_MEMORY_REGION.
>>
>> When the guest proxy reads from the winsrv socket, it will get a FD that
>> wraps the buffer referenced above.
>>
>> When the client reads from the guest proxy, it would get a FD that
>> references that same buffer and would mmap it. At that point, the client is
>> reading from the same physical pages where the compositor wrote to.
> 
> Hmm.  I allways assumed the wayland client allocates the buffers, not
> the server.  Is that wrong?
> 
> What is your plan for 3d acceleration support?
> 
>> To be clear, I'm not against solving this via some form of restricted FD
>> passing in virtio-vsock, but Stefan (added to CC) thought that it would be
>> cleaner to do it all within virtio-gpu.
> 
> Well, when targeting 3d acceleration it makes alot of sense to use
> virtio-gpu.  And it makes sense to have 2d and 3d modes work as simliar
> as possible.  That is not the direction you are taking with your
> proposal though ...
> 
> If you don't plan for 3d support I'm wondering whenever virtio-gpu is a
> good pick.  Mapping trickery aside, you wouldn't get linear buffers
> which can easily be shared between host and guest, because guest buffers
> are not required to be linear in guest physical memory.  One copy will
> be needed, from (scattered) guest physical memory buffer to (linear)
> host buffer.
> 
> One possible alternative would be to build on stdvga.  It has a pci
> memory bar, it has a drm driver (bochs) which allows allocating drm
> buffers in that bar.  They are linear buffers in both guest physical and
> host virtual memory.  If we add an option to qemu to allocate the memory
> bar in sysv shared memory it can easily be exported to other processes
> on the host.  The wayland client in the guest can map it directly too,
> it only needs to create a drm buffer and mmap it.  You can get zero-copy
> without having to play mapping tricks.
> 
> cheers,
>    Gerd
> 
> 

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