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

From: Dongwon Kim
Subject: Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device
Date: Fri, 6 Apr 2018 11:57:46 -0700
User-agent: Mutt/1.5.24 (2015-08-30)

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

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.

3. 1-level references - when using grant-table for sharing pages, there will
be same # of refs (each 8 byte) as # of shared pages, which is passed to
the userspace to be shared with importing VM in case of xen-zcopy. Compared
to this, hyper_dmabuf does multiple level addressing to generate only one
reference id that represents all shared pages.

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.

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.

> >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


> >
> >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

> >
> >cheers,
> >   Gerd
> >
> Thank you,
> Oleksandr
> P.S. Sorry for making your original mail thread to discuss things much
> broader than your RFC...

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