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

From: Daniel Vetter
Subject: Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device
Date: Mon, 16 Apr 2018 11:32:49 +0200

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
>>>>>> wrote:
>>>>>>> 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:
>>>>>>>>>>      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
>>>>>> Another thing is danvet was kind of against to the idea of importing
>>>>>> existing
>>>>>> dmabuf/prime buffer and forward it to the other domain due to
>>>>>> synchronization
>>>>>> 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
>>>>>> this
>>>>>> 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
>>>>> user-space
>>>>> to make xen-front allocate the buffers (make it exporter), but what we
>>>>> try
>>>>> to avoid is to change user-space which in normal world would have
>>>>> remain
>>>>> unchanged otherwise.
>>>>> So, I do think we have to support this use-case and just have to
>>>>> understand
>>>>> 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
>>>> hypervisor).
>>>> 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
>>>> over
>>>>     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
>>>> need
>>>>     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
>> anywhere.
>> 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

Also I'm not clear in your example which step happens where (guest 1/2
or hypervisor)?

> 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
> handle_local
> 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
>>>> another
>>>> 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:
>>>> https://dri.freedesktop.org/docs/drm/gpu/drm-uapi.html#open-source-userspace-requirements
>>>> 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
>>>>>>>> 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.
>>>>>> Yeah, understood xen-zcopy works ok with your use case. But I am just
>>>>>> curious
>>>>>> 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
>>>> the
>>>> replies while catching up.
>>> I was just referring to accounting stuff already implemented in the DRM
>>> core,
>>> 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
>>>>>>> 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.
>>>>>>>>     Compared
>>>>>>>> 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).
>>>>>> 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
>>>>>> general
>>>>>> memory sharing use case or is it jsut for xen-display (in current code
>>>>>> base)?
>>>>> 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
>>>>> we
>>>>> 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
>>>>> defines
>>>>> all the required constants for requests, responses, defines message
>>>>> formats,
>>>>> state diagrams etc. all at one place. Of course this protocol must not
>>>>> be
>>>>> Xen specific, but be OS/hypervisor agnostic.
>>>>> Having that will trigger a new round of discussion, so we have it all
>>>>> designed
>>>>> 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
>>>>> be
>>>>> number
>>>>> of kernel-space users as well.
>>>> Again, why do you want to create new uapi for this? Given the very
>>>> strict
>>>> requirements we have for new uapi (see above link), it's the toughest
>>>> way
>>>> 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
>>> example,
>>> some other para-virtual driver can export/import dma-buf, not only
>>> user-space.
>> 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

Please be more specific in what you mean instead of just "backend",
that's really confusing.

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
it. 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
>>>> some
>>>> implementation improvements, and stuff to make sure xen-front actually
>>>> implements the real uapi semantics instead of its own), and why I'm
>>>> asking
>>>> much more questions on this stuff here.
>>>>>>>> 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
>>>>>>>> visit:
>>>>>>>> https://github.com/downor/linux_hyper_dmabuf_test/tree/xen/simple_export
>>>>>>> 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
>>>>>>>> solution.
>>>>>>> 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)
>>>>>> I think it is possible to use your functions for memory sharing part
>>>>>> in
>>>>>> hyper_dmabuf's backend (this 'backend' means the layer that does page
>>>>>> sharing
>>>>>> 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
>>>>>> our
>>>>>> discussion on the common API layer that can cover any dmabuf sharing
>>>>>> cases.
>>>>> 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,
>>>>>>> Oleksandr
>>>>>>> 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
>>>>> mail
>>>>> 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
>>>> have
>>>> 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
> flip.

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?).
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

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