[virtio-dev] Re: [RFC] Upstreaming virtio-wayland (or an alternative)

[Boris Brezillon <boris.brezillon@collabora.com>]

Hi Stéphane,

On Mon, 10 Feb 2020 12:01:02 -0800
Stéphane Marchesin <marcheu@chromium.org> wrote:

> On Fri, Feb 7, 2020 at 9:28 AM Boris Brezillon <
> boris.brezillon@collabora.com> wrote:  
> 
> > Hello everyone,
> >
> > I recently took over Tomeu's task of upstreaming virtio-wayland. After
> > spending quite a bit of time collecting information from his different
> > attempts [1][2] I wanted to sync with all the people that were involved
> > in the previous discussions (if I missed some of them, feel free to add
> > them back).
> >
> > The goal here is to get a rough idea of the general direction this
> > should take so I can start implementing a PoC and see if it fits
> > everyone's needs.
> >
> > virtio-wayland [3] started as a solution to pass wayland messages
> > between host and guests so the guest can execute wayland apps whose
> > surface buffers are passed to the wayland compositor running on the
> > host. While this was its primary use case, I've heard it's been used to
> > transport other protocols. And that's not surprising, when looking at
> > the code I noticed it was providing a protocol-agnostic message passing
> > interface between host and guests, similar to what VSOCK provides but
> > with FD passing as an extra feature.
> >
> > Based on all previous discussions, I could identify 3 different
> > approaches:
> >
> >     1/ Use VSOCK and extend it to support passing (some) FDs
> >     2/ Use a user space VSOCK-based proxy that's in charge of
> >        a/ passing regular messages
> >        b/ passing specific handles to describe objects shared
> >           between host and guest (most focus has been on dmabufs as
> >           this is what we really care about for the gfx use case,
> >           but other kind of FDs can be emulated through a
> >           VSOCK <-> UNIX_SOCK bridging)
> >     3/ Have a dedicated kernel space solution that provides features
> >        exposed by #1 but through a virtio device interface (basically
> >        what virtio-wayland does today)
> >
> > Each of them has its pros and cons, which I'll try to sum-up (please
> > correct me if I'm wrong, and add new things if you think they are
> > missing).
> >
> > #1 might require extra care if we want to make it safe, as pointed
> > out by Stefan here [4] (but I wonder if the problem is not the same
> > for a virtio-wayland based solution). Of course you also need a bit of
> > infrastructure to register FD <-> VFD mappings (VFD being a virtual
> > file descriptor that's only used as unique IDs identifying the resource
> > backed by the local FD). FD <-> VFD mappings would have to be created
> > by the subsystem in charge of the object backing the FD (virtio-gpu for
> > exported GEM buffers, virtio-vdec for video buffers, vsock for unix
> > sockets if we decide to bridge unix and vsock sockets to make it
> > transparent, ...). The FD <-> VFD mapping would also have to be created
> > on the host side, probably by the virtio device implementation
> > (virglrenderer for GEM bufs for instance), which means host and guest
> > need a way to inform the other end that a new FD <-> VFD mapping has
> > been created so the other end can create a similar mapping (I guess this
> > requires extra device-specific commands to work). Note that this
> > solution doesn't look so different from the virtio-dmabuf [5] approach
> > proposed by Gerd a few months back, it's just extended to be a global
> > VFD <-> FD registry instead of a dmabuf <-> unique-handle one. One
> > great thing about this approach is that we can re-use it for any kind
> > of FD sharing, not just dmabufs.
> >
> > #2 is a bit challenging, since it requires the proxy to know about all
> > possible kind of FDs and do a FD <-> unique handle conversion with some
> > help from the subsystem backing the FD. For dmabufs, that means we
> > need to know who created the dmabuf, or assume that only one device is
> > used for all allocations (virtio-gpu?). AFAIU, there's also a security
> > issue as one could pass random (but potentially valid) handles to the
> > host proxy (pointed out by Tomasz [6]).
> >
> > #3 is pretty similar to #1 in its design except that, instead of using
> > the VSOCK infrastructure it's using a new type of virtio device. I
> > guess it has the same pros and cons #1 has, and the name should probably
> > be changed to reflect the fact that it can transmit any kind of data not
> > just wayland.
> >
> > This is just a high level view of the problem and the solutions proposed
> > by various people over the years. I'm sure I'm missing tons of details
> > and don't realize yet all the complexity behind solution #1, but looking
> > at this summary, I wonder if I should investigate this solution in
> > priority. An alternative could be to rebrand virtio-wayland, but as I
> > said, it's close enough to VSOCK to try to merge the missing features
> > in VSOCK instead. This being said, I'm not yet set on any of those
> > solutions, and the point of this email is to see with all of you which
> > option I should investigate first.
> >
> > Note that option #3 is already implemented (would have to be polished
> > for upstream), IIRC option #2 has been partially implemented by Tomeu
> > but I'm not sure it was finished, and option #1 has just been discussed
> > so far [2].
> >
> > Any feedback/comment is welcome.
> >  
> 
> One of the key things I feel we need is for the host-side to be aware of
> what's going on with the life cycle of the buffers in one single place.
> 
> Today we are making suboptimal buffer allocations because we have to
> pessimistically assume that all buffers with the display flag are going to
> be displayed (even though they might not be used that way). Furthermore, we
> can't reallocate buffers on the fly to fit changing usage models, for
> example to go to different overlays. If virglrenderer knew about the usage
> in real time, it could make more optimal allocation decisions. Today this
> is one of the main aspects limiting the performance we can get out of our
> VMs.

Could you elaborate on that specific aspect? Where/when the
re-allocation/copy would happen (I suspect it's done at
wl_surface.commit() time but I'm not sure). I was also wondering if
this wp_linux_dmabuf_hints extension [1] couldn't help getting the most
optimal format/modifier without requiring this re-allocation/copy on
the host side.

Regards,

Boris

[1]https://gitlab.freedesktop.org/wayland/wayland-protocols/merge_requests/8

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