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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: Mon, 16 Apr 2018 13:14:17 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 04/16/2018 12:32 PM, Daniel Vetter wrote:
On Mon, Apr 16, 2018 at 10:22 AM, Oleksandr Andrushchenko
<address@hidden> wrote:
On 04/16/2018 10:43 AM, Daniel Vetter wrote:
On Mon, Apr 16, 2018 at 10:16:31AM +0300, Oleksandr Andrushchenko wrote:
On 04/13/2018 06:37 PM, Daniel Vetter wrote:
On Wed, Apr 11, 2018 at 08:59:32AM +0300, Oleksandr Andrushchenko wrote:
On 04/10/2018 08:26 PM, Dongwon Kim wrote:
On Tue, Apr 10, 2018 at 09:37:53AM +0300, Oleksandr Andrushchenko
On 04/06/2018 09:57 PM, Dongwon Kim wrote:
On Fri, Apr 06, 2018 at 03:36:03PM +0300, Oleksandr Andrushchenko
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
away into a separate kernel modules even though most of the actual
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 ->
underlying pages and get those shared -> return references for
shared pages
Another thing is danvet was kind of against to the idea of importing
dmabuf/prime buffer and forward it to the other domain due to
issues. He proposed to make hyper_dmabuf only work as an exporter so
that it
can have a full control over the buffer. I think we need to talk about
further as well.
Yes, I saw this. But this limits the use-cases so much.
For instance, running Android as a Guest (which uses ION to allocate
buffers) means that finally HW composer will import dma-buf into
the DRM driver. Then, in case of xen-front for example, it needs to be
shared with the backend (Host side). Of course, we can change
to make xen-front allocate the buffers (make it exporter), but what we
to avoid is to change user-space which in normal world would have
unchanged otherwise.
So, I do think we have to support this use-case and just have to
the complexity.
Erm, why do you need importer capability for this use-case?

guest1 -> ION -> xen-front -> hypervisor -> guest 2 -> xen-zcopy exposes
that dma-buf -> import to the real display hw

No where in this chain do you need xen-zcopy to be able to import a
dma-buf (within linux, it needs to import a bunch of pages from the

Now if your plan is to use xen-zcopy in the guest1 instead of xen-front,
then you indeed need to import.
This is the exact use-case I was referring to while saying
we need to import on Guest1 side. If hyper-dmabuf is so
generic that there is no xen-front in the picture, then
it needs to import a dma-buf, so it can be exported at Guest2 side.
    But that imo doesn't make sense:
- xen-front gives you clearly defined flip events you can forward to the
     hypervisor. xen-zcopy would need to add that again.
xen-zcopy is a helper driver which doesn't handle page flips
and is not a KMS driver as one might think of: the DRM UAPI it uses is
just to export a dma-buf as a PRIME buffer, but that's it.
Flipping etc. is done by the backend [1], not xen-zcopy.
    Same for
     hyperdmabuf (and really we're not going to shuffle struct dma_fence
     the wire in a generic fashion between hypervisor guests).

- xen-front already has the idea of pixel format for the buffer (and any
     other metadata). Again, xen-zcopy and hyperdmabuf lack that, would
     to add it shoehorned in somehow.
Again, here you are talking of something which is implemented in
Xen display backend, not xen-zcopy, e.g. display backend can
implement para-virtual display w/o xen-zcopy at all, but in this case
there is a memory copying for each frame. With the help of xen-zcopy
the backend feeds xen-front's buffers directly into Guest2 DRM/KMS or
Weston or whatever as xen-zcopy exports remote buffers as PRIME buffers,
thus no buffer copying is required.
Why do you need to copy on every frame for xen-front? In the above
pipeline, using xen-front I see 0 architectural reasons to have a copy

This seems to be the core of the confusion we're having here.
Ok, so I'll try to explain:
1. xen-front - produces a display buffer to be shown at Guest2
by the backend, shares its grant references with the backend
2. xen-front sends page flip event to the backend specifying the
buffer in question
3. Backend takes the shared buffer (which is only a buffer mapped into
backend's memory, it is not a dma-buf/PRIME one) and makes memcpy from
it to a local dumb/surface
Why do you even do that? The copying here I mean - why don't you just
directly scan out from the grant references you received through the
Probably the confusion comes from the fact that KVM and Xen
implement things differently (for example, on ARM we don't use QEMU at all).
Please see [1] and [2] for Xen frontend/backend placement in the picture.

WRT to [2] xen-front is a PV front-end driver running in guest OS
and Xen display backend is a user-space application running in Dom0
(in the picture [2] backend runs as a Dom0 kernel driver).
So, the para-virtualized device is not implemented in the hypervisor
itself, but as user/kernel-space pair in corresponding domains.
Thus, when xen-front shares grant references of the pages of the buffer
with the Xen display backend (user-space) the later can only map those
references into Dom0 memory to memcpy into some local display buffer/dumb.
Hence, hypervisor is not in the equation while actually implementing
para-virtual display device, e.g. it provides you with API to share/map
pages, but it won't be the entity which will implement actual page flips etc.
So, this is where xen-zcopy comes into the play (runs in Dom0):
it not only maps xen-front's grant references into Dom0, but also creates
a PRIME buffer, so this buffer can be used by other DRM devices/Weston
running in Dom0.
Also I'm not clear in your example which step happens where (guest 1/2
or hypervisor)?
Steps 1,2 - Guest2, kernel space
Steps 3-4 - Guest1, Dom0 user-space
The hypervisor here only provides transport and means to access buffers,
actual display/DRM related code is in xen-front and Dom0's display backend
4. Backend flips that local dumb buffer/surface

If I have a xen-zcopy helper driver then I can avoid doing step 3):
1) 2) remain the same as above
3) Initially for a new display buffer, backend calls xen-zcopy to create
a local PRIME buffer from the grant references provided by the xen-front
via displif protocol [1]: we now have handle_zcopy
4) Backend exports this PRIME with HANDLE_TO_FD from xen-zcopy and imports
it into Weston-KMS/DRM or real HW DRM driver with FD_TO_HANDLE: we now have
5) On page flip event backend flips local PRIME: uses handle_local for flips

Ofc you won't be able to shovel sound or media stream data over to
guest like this, but that's what you have xen-v4l and xen-sound or
whatever else for. Trying to make a new uapi, which means userspace must
be changed for all the different use-case, instead of reusing standard
linux driver uapi (which just happens to send the data to another
hypervisor guest instead of real hw) imo just doesn't make much sense.

Also, at least for the gpu subsystem: Any new uapi must have full
userspace available for it, see:


Adding more uapi is definitely the most painful way to fix a use-case.
Personally I'd go as far and also change the xen-zcopy side on the
receiving guest to use some standard linux uapi. E.g. you could write an
output v4l driver to receive the frames from guest1.
So, we now know that xen-zcopy was not meant to handle page flips,
but to implement new UAPI to let user-space create buffers either
from Guest2 grant references (so it can be exported to Guest1) or
other way round, e.g. create (from Guest1 grant references to export to
Guest 2). For that reason it adds 2 IOCTLs: create buffer from grefs
or produce grefs for the buffer given.
One additional IOCTL is to wait for the buffer to be released by
Guest2 user-space.
That being said, I don't quite see how v4l can be used here to implement
UAPI I need.
Under the assumption that you can make xen-front to zerocopy for the
kernel->hypervisor path, v4l could be made to work for the
hypervisor->kernel side of the pipeline.

But it sounds like we have a confusion already on why or why not xen-front
can or cannot do zerocopy.
xen-front provides an array of grant references to Guest2 (backend).
It's up to backend what it does with those grant references
which at Guest2 side are not PRIME or dma-buf, but just a set of pages.
This is xen-zcopy which turns these pages into a PRIME. When this is done
backend can now tell DRM drivers to use the buffer in DRM terms.

danvet, can you comment on this topic?

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

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

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
Yeah, understood xen-zcopy works ok with your use case. But I am just
if it is ok not to have any inter-domain synchronization in this
sharing model.
The synchronization is done with displif protocol [1]
The buffer being shared is technically dma-buf and originator needs to
be able
to keep track of it.
As I am working in DRM terms the tracking is done by the DRM core
for me for free. (This might be one of the reasons Daniel sees DRM
based implementation fit very good from code-reuse POV).
Hm, not sure what tracking you refer to here all ... I got lost in all
replies while catching up.

I was just referring to accounting stuff already implemented in the DRM
so I don't have to worry about doing the same for buffers to understand
when they are released etc.
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
You are right. Thanks for correction.;)

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
Xen display protocol [2]. So, effectively the backend extracts
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
between VMs
(and the gref directory is a single-linked list of shared pages
of the grefs of the buffer).
ok, good to know. I will look into its implementation in more details
but is
this gref directory (chained grefs) something that can be used for any
memory sharing use case or is it jsut for xen-display (in current code
Not to mislead you: one grant ref is passed via displif protocol,
but the page it's referencing contains the rest of the grant refs.

As to if this can be used for any memory: yes. It is the same for
sndif and displif Xen protocols, but defined twice as strictly speaking
sndif and displif are two separate protocols.

While reviewing your RFC v2 one of the comments I had [2] was that if
can start from defining such a generic protocol for hyper-dmabuf.
It can be a header file, which not only has the description part
(which then become a part of Documentation/...rst file), but also
all the required constants for requests, responses, defines message
state diagrams etc. all at one place. Of course this protocol must not
Xen specific, but be OS/hypervisor agnostic.
Having that will trigger a new round of discussion, so we have it all
and discussed before we start implementing.

Besides the protocol we have to design UAPI part as well and make sure
the hyper-dmabuf is not only accessible from user-space, but there will
of kernel-space users as well.
Again, why do you want to create new uapi for this? Given the very
requirements we have for new uapi (see above link), it's the toughest
to get any kind of support in.
I do understand that adding new UAPI is not good for many reasons.
But here I was meaning that current hyper-dmabuf design is
only user-space oriented, e.g. it provides number of IOCTLs to do all
the work. But I need a way to access the same from the kernel, so, for
some other para-virtual driver can export/import dma-buf, not only
If you need an import-export helper library, just merge it. Do not attach
any uapi to it, just the internal helpers.

Much, much, much easier to land.
This can be done, but again, I will need some entity which
backend may use to convert xen-front's grant references into
a PRIME buffer, hence there is UAPI for that. In other words,
I'll need a thiner xen-zcopy which will implement the same UAPI
and use that library for Xen related stuff.

The confusion may also come from the fact that the backend is
a user-space application, not a kernel module (we have 2 modes
of its operation as of now: DRM master or Weston client), so
it needs a way to talk to the kernel.
So this is entirely a means to implement the virtual xen device in
dom0 (or whichever guest implements it)?

I'm externally confused about what you mean with "backend", since
xen-front also has backend code. But that backend code lives in the
same guest os image (afaict at least), since it does direct function
xen-front has no backend code, but only has code which allows it
to create a dumb buffer from the grant references provided by the
Please be more specific in what you mean instead of just "backend",
that's really confusing.
Hope [2] better explains this

But essentially we're talking about the equivalent of what qemu does
for kvm, and that's entirely not my problem. Not really a gpu
subsystem problem I think. Just talk with the xen hypervisor people
about how exactly they want to go about converting grant tables to
dma-buf, so that your virtual hw backend in userspace can make use of
The problem here is that the display backend then will need
to talk to DRM. And what is the UAPI for that? Right, PRIME
And then merge it somewhere in the xen directories. Since the
grant tables and everything is very xen specific, I don't think
there's much point in trying to have a fake generic uapi that pretends
to work on other hypervisors, as long as they're Xen :-)

And you probably have no need for all the caching/general book-keeping
drm_prime does (it's all in userspace I guess, except for the magic
conversion from grant references to a dma_buf). So there's no point
trying to reuse code in drm_prime.c.

Also, this should make it tons easier to reuse xen-zcopy for
sound/wireless/v4l backends.

That's why I had essentially zero big questions for xen-front (except
implementation improvements, and stuff to make sure xen-front actually
implements the real uapi semantics instead of its own), and why I'm
much more questions on this stuff here.

4. inter VM messaging (hype_dmabuf only) - hyper_dmabuf has inter-vm
communication defined for dmabuf synchronization and private data
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
notified when newdmabuf is exported from other VM - uevent can be
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
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
, notification and dma-buf synchronization then re-structured it to
front-end and back-end (this made things more compicated..) since
was not our only target. Also, we thought passing the reference for
buffer (hyper_dmabuf_id) is not secure so added uvent mechanism

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,

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
on top of xen-zcopy.
Hm, I can imagine that: xen-zcopy could be a library code for
in terms of implementing all that page sharing fun in multiple
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
for bringing that up Oleksandr). However, the question is once we
somehow, can xen-zcopy's usecase use the standard API that
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
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
so common code of the above drivers can use the same code w/o code
I think it is possible to use your functions for memory sharing part
hyper_dmabuf's backend (this 'backend' means the layer that does page
and inter-vm communication with xen-specific way.), so why don't we
work on
"Xen helper library for sharing big buffers" first while we continue
discussion on the common API layer that can cover any dmabuf sharing

Well, I would love we reuse the code that I have, but I also
understand that it was limited by my use-cases. So, I do not
insist we have to ;)
If we start designing and discussing hyper-dmabuf protocol we of course
can work on this helper library in parallel.
Imo code reuse is overrated. Adding new uapi is what freaks me out here

If we end up with duplicated implementations, even in upstream, meh, not
great, but also ok. New uapi, and in a similar way, new hypervisor api
like the dma-buf forwarding that hyperdmabuf does is the kind of thing
that will lock us in for 10+ years (if we make a mistake).

Thank you,

P.S. All, is it a good idea to move this out of udmabuf thread into a
dedicated one?
Either way is fine with me.
So, if you can start designing the protocol we may have a dedicated
thread for that. I will try to help with the protocol as much as I can
Please don't start with the protocol. Instead start with the concrete
use-cases, and then figure out why exactly you need new uapi. Once we
that answered, we can start thinking about fleshing out the details.
On my side there are only 2 use-cases, Guest2 only:
1. Create a PRIME (dma-buf) from grant references
2. Create grant references from PRIME (dma-buf)
So these grant references, are those userspace visible things?
Yes, the user-space backend receives those from xen-front via [1]

I thought
the grant references was just the kernel/hypervisor internal magic to make
this all work?
So, I can map the grant references from user-space, but I won't
be able to turn those into a PRIME buffer. So, the only use of those
w/o xen-zcopy is to map grant refs and copy into real HW dumb on every page
Ok, that explains. I thought your current xen-side implementation for
xen-front is already making all that stuff happen. But I'm still not
sure given all the confusing talk about back-end we have in these
threads (hyperdmabuf people also talked about different backends for
different hypervisors, I guess that's a different kind of backend?).
Hope the explanation above makes it all clearer.
Please let me know if you still want me to elaborate more
[1] https://wiki.xen.org/wiki/Paravirtualization_(PV)
[2] https://wiki.xen.org/wiki/File:XenPV.png

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