Messages in this thread | | | Subject | Re: [PATCH v3 1/2] drm/virtio: Add window server support | From | Tomeu Vizoso <> | Date | Tue, 6 Feb 2018 13:41:52 +0100 |
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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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