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Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device

From: Oleksandr Andrushchenko
Subject: Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device
Date: Tue, 10 Apr 2018 09:37:53 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 04/06/2018 09:57 PM, Dongwon Kim wrote:
On Fri, Apr 06, 2018 at 03:36:03PM +0300, Oleksandr Andrushchenko wrote:
On 04/06/2018 02:57 PM, Gerd Hoffmann wrote:

I fail to see any common ground for xen-zcopy and udmabuf ...
Does the above mean you can assume that xen-zcopy and udmabuf
can co-exist as two different solutions?
Well, udmabuf route isn't fully clear yet, but yes.

See also gvt (intel vgpu), where the hypervisor interface is abstracted
away into a separate kernel modules even though most of the actual vgpu
emulation code is common.
Thank you for your input, I'm just trying to figure out
which of the three z-copy solutions intersect and how much
And what about hyper-dmabuf?
xen z-copy solution is pretty similar fundamentally to hyper_dmabuf
in terms of these core sharing feature:

1. the sharing process - import prime/dmabuf from the producer -> extract
underlying pages and get those shared -> return references for shared pages

2. the page sharing mechanism - it uses Xen-grant-table.

And to give you a quick summary of differences as far as I understand
between two implementations (please correct me if I am wrong, Oleksandr.)

1. xen-zcopy is DRM specific - can import only DRM prime buffer
while hyper_dmabuf can export any dmabuf regardless of originator
Well, this is true. And at the same time this is just a matter
of extending the API: xen-zcopy is a helper driver designed for
xen-front/back use-case, so this is why it only has DRM PRIME API

2. xen-zcopy doesn't seem to have dma-buf synchronization between two VMs
while (as danvet called it as remote dmabuf api sharing) hyper_dmabuf sends
out synchronization message to the exporting VM for synchronization.
This is true. Again, this is because of the use-cases it covers.
But having synchronization for a generic solution seems to be a good idea.

3. 1-level references - when using grant-table for sharing pages, there will
be same # of refs (each 8 byte)
To be precise, grant ref is 4 bytes
as # of shared pages, which is passed to
the userspace to be shared with importing VM in case of xen-zcopy.
The reason for that is that xen-zcopy is a helper driver, e.g.
the grant references come from the display backend [1], which implements
Xen display protocol [2]. So, effectively the backend extracts references
from frontend's requests and passes those to xen-zcopy as an array
of refs.
to this, hyper_dmabuf does multiple level addressing to generate only one
reference id that represents all shared pages.
In the protocol [2] only one reference to the gref directory is passed between VMs (and the gref directory is a single-linked list of shared pages containing all
of the grefs of the buffer).

4. inter VM messaging (hype_dmabuf only) - hyper_dmabuf has inter-vm msg
communication defined for dmabuf synchronization and private data (meta
info that Matt Roper mentioned) exchange.
This is true, xen-zcopy has no means for inter VM sync and meta-data,
simply because it doesn't have any code for inter VM exchange in it,
e.g. the inter VM protocol is handled by the backend [1].

5. driver-to-driver notification (hyper_dmabuf only) - importing VM gets
notified when newdmabuf is exported from other VM - uevent can be optionally
generated when this happens.

6. structure - hyper_dmabuf is targetting to provide a generic solution for
inter-domain dmabuf sharing for most hypervisors, which is why it has two
layers as mattrope mentioned, front-end that contains standard API and backend
that is specific to hypervisor.
Again, xen-zcopy is decoupled from inter VM communication
No idea, didn't look at it in detail.

Looks pretty complex from a distant view.  Maybe because it tries to
build a communication framework using dma-bufs instead of a simple
dma-buf passing mechanism.
we started with simple dma-buf sharing but realized there are many
things we need to consider in real use-case, so we added communication
, notification and dma-buf synchronization then re-structured it to
front-end and back-end (this made things more compicated..) since Xen
was not our only target. Also, we thought passing the reference for the
buffer (hyper_dmabuf_id) is not secure so added uvent mechanism later.

Yes, I am looking at it now, trying to figure out the full story
and its implementation. BTW, Intel guys were about to share some
test application for hyper-dmabuf, maybe I have missed one.
It could probably better explain the use-cases and the complexity
they have in hyper-dmabuf.
One example is actually in github. If you want take a look at it, please

Thank you, I'll have a look
Like xen-zcopy it seems to depend on the idea that the hypervisor
manages all memory it is easy for guests to share pages with the help of
the hypervisor.
So, for xen-zcopy we were not trying to make it generic,
it just solves display (dumb) zero-copying use-cases for Xen.
We implemented it as a DRM helper driver because we can't see any
other use-cases as of now.
For example, we also have Xen para-virtualized sound driver, but
its buffer memory usage is not comparable to what display wants
and it works somewhat differently (e.g. there is no "frame done"
event, so one can't tell when the sound buffer can be "flipped").
At the same time, we do not use virtio-gpu, so this could probably
be one more candidate for shared dma-bufs some day.
   Which simply isn't the case on kvm.

hyper-dmabuf and xen-zcopy could maybe share code, or hyper-dmabuf build
on top of xen-zcopy.
Hm, I can imagine that: xen-zcopy could be a library code for hyper-dmabuf
in terms of implementing all that page sharing fun in multiple directions,
e.g. Host->Guest, Guest->Host, Guest<->Guest.
But I'll let Matt and Dongwon to comment on that.
I think we can definitely collaborate. Especially, maybe we are using some
outdated sharing mechanism/grant-table mechanism in our Xen backend (thanks
for bringing that up Oleksandr). However, the question is once we collaborate
somehow, can xen-zcopy's usecase use the standard API that hyper_dmabuf
provides? I don't think we need different IOCTLs that do the same in the final

If you think of xen-zcopy as a library (which implements Xen
grant references mangling) and DRM PRIME wrapper on top of that
library, we can probably define proper API for that library,
so both xen-zcopy and hyper-dmabuf can use it. What is more, I am
about to start upstreaming Xen para-virtualized sound device driver soon,
which also uses similar code and gref passing mechanism [3].
(Actually, I was about to upstream drm/xen-front, drm/xen-zcopy and
snd/xen-front and then propose a Xen helper library for sharing big buffers,
so common code of the above drivers can use the same code w/o code duplication)

Thank you,

P.S. All, is it a good idea to move this out of udmabuf thread into a dedicated one?

Thank you,

P.S. Sorry for making your original mail thread to discuss things much
broader than your RFC...

[1] https://github.com/xen-troops/displ_be
[2] https://elixir.bootlin.com/linux/v4.16-rc7/source/include/xen/interface/io/displif.h#L484 [3] https://elixir.bootlin.com/linux/v4.16-rc7/source/include/xen/interface/io/sndif.h

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