Re: guest / host buffer sharing ...

[Dylan Reid <dgreid@chromium.org>]

On Thu, Nov 21, 2019 at 4:59 PM Tomasz Figa <tfiga@chromium.org> wrote:
>
> On Thu, Nov 21, 2019 at 6:41 AM Geoffrey McRae <geoff@hostfission.com> wrote:
> >
> >
> >
> > On 2019-11-20 23:13, Tomasz Figa wrote:
> > > Hi Geoffrey,
> > >
> > > On Thu, Nov 7, 2019 at 7:28 AM Geoffrey McRae <geoff@hostfission.com>
> > > wrote:
> > >>
> > >>
> > >>
> > >> On 2019-11-06 23:41, Gerd Hoffmann wrote:
> > >> > On Wed, Nov 06, 2019 at 05:36:22PM +0900, David Stevens wrote:
> > >> >> > (1) The virtio device
> > >> >> > =====================
> > >> >> >
> > >> >> > Has a single virtio queue, so the guest can send commands to register
> > >> >> > and unregister buffers.  Buffers are allocated in guest ram.  Each buffer
> > >> >> > has a list of memory ranges for the data. Each buffer also has some
> > >> >>
> > >> >> Allocating from guest ram would work most of the time, but I think
> > >> >> it's insufficient for many use cases. It doesn't really support things
> > >> >> such as contiguous allocations, allocations from carveouts or <4GB,
> > >> >> protected buffers, etc.
> > >> >
> > >> > If there are additional constrains (due to gpu hardware I guess)
> > >> > I think it is better to leave the buffer allocation to virtio-gpu.
> > >>
> > >> The entire point of this for our purposes is due to the fact that we
> > >> can
> > >> not allocate the buffer, it's either provided by the GPU driver or
> > >> DirectX. If virtio-gpu were to allocate the buffer we might as well
> > >> forget
> > >> all this and continue using the ivshmem device.
> > >
> > > I don't understand why virtio-gpu couldn't allocate those buffers.
> > > Allocation doesn't necessarily mean creating new memory. Since the
> > > virtio-gpu device on the host talks to the GPU driver (or DirectX?),
> > > why couldn't it return one of the buffers provided by those if
> > > BIND_SCANOUT is requested?
> > >
> >
> > Because in our application we are a user-mode application in windows
> > that is provided with buffers that were allocated by the video stack in
> > windows. We are not using a virtual GPU but a physical GPU via vfio
> > passthrough and as such we are limited in what we can do. Unless I have
> > completely missed what virtio-gpu does, from what I understand it's
> > attempting to be a virtual GPU in its own right, which is not at all
> > suitable for our requirements.
>
> Not necessarily. virtio-gpu in its basic shape is an interface for
> allocating frame buffers and sending them to the host to display.
>
> It sounds to me like a PRIME-based setup similar to how integrated +
> discrete GPUs are handled on regular systems could work for you. The
> virtio-gpu device would be used like the integrated GPU that basically
> just drives the virtual screen. The guest component that controls the
> display of the guest (typically some sort of a compositor) would
> allocate the frame buffers using virtio-gpu and then import those to
> the vfio GPU when using it for compositing the parts of the screen.
> The parts of the screen themselves would be rendered beforehand by
> applications into local buffers managed fully by the vfio GPU, so
> there wouldn't be any need to involve virtio-gpu there. Only the
> compositor would have to be aware of it.
>
> Of course if your guest is not Linux, I have no idea if that can be
> handled in any reasonable way. I know those integrated + discrete GPU
> setups do work on Windows, but things are obviously 100% proprietary,
> so I don't know if one could make them work with virtio-gpu as the
> integrated GPU.
>
> >
> > This discussion seems to have moved away completely from the original
> > simple feature we need, which is to share a random block of guest
> > allocated ram with the host. While it would be nice if it's contiguous
> > ram, it's not an issue if it's not, and with udmabuf (now I understand
> > it) it can be made to appear contigous if it is so desired anyway.
> >
> > vhost-user could be used for this if it is fixed to allow dynamic
> > remapping, all the other bells and whistles that are virtio-gpu are
> > useless to us.
> >
>
> As far as I followed the thread, my impression is that we don't want
> to have an ad-hoc interface just for sending memory to the host. The
> thread was started to look for a way to create identifiers for guest
> memory, which proper virtio devices could use to refer to the memory
> within requests sent to the host.
>
> That said, I'm not really sure if there is any benefit of making it
> anything other than just the specific virtio protocol accepting
> scatterlist of guest pages directly.
>
> Putting the ability to obtain the shared memory itself, how do you
> trigger a copy from the guest frame buffer to the shared memory?

Adding Zach for more background on virtio-wl particular use cases.