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Subject: Re: guest / host buffer sharing ...

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?

> Our use case is niche, and the state of things may change if vendors
> like
> AMD follow through with their promises and give us SR-IOV on consumer
> GPUs, but even then we would still need their support to achieve the
> same
> results as the same issue would still be present.
> Also don't forget that QEMU already has a non virtio generic device
> (IVSHMEM). The only difference is, this device doesn't allow us to
> attain
> zero-copy transfers.
> Currently IVSHMEM is used by two projects that I am aware of, Looking
> Glass and SCREAM. While Looking Glass is solving a problem that is out
> of
> scope for QEMU, SCREAM is working around the audio problems in QEMU that
> have been present for years now.
> While I don't agree with SCREAM being used this way (we really need a
> virtio-sound device, and/or intel-hda needs to be fixed), it again is an
> example of working around bugs/faults/limitations in QEMU by those of us
> that are unable to fix them ourselves and seem to have low priority to
> the
> QEMU project.
> What we are trying to attain is freedom from dual boot Linux/Windows
> systems, not migrate-able enterprise VPS configurations. The Looking
> Glass project has brought attention to several other bugs/problems in
> QEMU, some of which were fixed as a direct result of this project (i8042
> race, AMD NPT).
> Unless there is another solution to getting the guest GPUs frame-buffer
> back to the host, a device like this will always be required. Since the
> landscape could change at any moment, this device should not be a LG
> specific device, but rather a generic device to allow for other
> workarounds like LG to be developed in the future should they be
> required.
> Is it optimal? no
> Is there a better solution? not that I am aware of
> >
> > virtio-gpu can't do that right now, but we have to improve virtio-gpu
> > memory management for vulkan support anyway.
> >
> >> > properties to carry metadata, some fixed (id, size, application), but
> >>
> >> What exactly do you mean by application?
> >
> > Basically some way to group buffers.  A wayland proxy for example would
> > add a "application=wayland-proxy" tag to the buffers it creates in the
> > guest, and the host side part of the proxy could ask qemu (or another
> > vmm) to notify about all buffers with that tag.  So in case multiple
> > applications are using the device in parallel they don't interfere with
> > each other.
> >
> >> > also allow free form (name = value, framebuffers would have
> >> > width/height/stride/format for example).
> >>
> >> Is this approach expected to handle allocating buffers with
> >> hardware-specific constraints such as stride/height alignment or
> >> tiling? Or would there need to be some alternative channel for
> >> determining those values and then calculating the appropriate buffer
> >> size?
> >
> > No parameter negotiation.
> >
> > cheers,
> >   Gerd

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