Message-ID: <89b044da-8986-d361-f557-26331a3de317@gmail.com>
Date:   Mon, 16 Apr 2018 13:14:17 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Dongwon Kim <dongwon.kim@...el.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        qemu-devel@...gnu.org,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Gerd Hoffmann <kraxel@...hat.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        "Oleksandr_Andrushchenko@...m.com" <Oleksandr_Andrushchenko@...m.com>
Subject: Re: [RfC PATCH] Add udmabuf misc device

On 04/16/2018 12:32 PM, Daniel Vetter wrote:
> On Mon, Apr 16, 2018 at 10:22 AM, Oleksandr Andrushchenko
> <andr2000@...il.com> 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
> hypervisor?
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
>> 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
> calls.
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
backend.
> 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
> it.
The problem here is that the display backend then will need
to talk to DRM. And what is the UAPI for that? Right, PRIME
buffers.
> 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?).
Hope the explanation above makes it all clearer.
Please let me know if you still want me to elaborate more
> -Daniel
[1] https://wiki.xen.org/wiki/Paravirtualization_(PV)
[2] https://wiki.xen.org/wiki/File:XenPV.png

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