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Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver

On Thu, Apr 19, 2018 at 11:14:02AM +0300, Oleksandr Andrushchenko wrote:
> On 04/18/2018 08:01 PM, Dongwon Kim wrote:
> >On Wed, Apr 18, 2018 at 09:38:39AM +0300, Oleksandr Andrushchenko wrote:
> >>On 04/17/2018 11:57 PM, Dongwon Kim wrote:
> >>>On Tue, Apr 17, 2018 at 09:59:28AM +0200, Daniel Vetter wrote:
> >>>>On Mon, Apr 16, 2018 at 12:29:05PM -0700, Dongwon Kim wrote:
> >>>>>Yeah, I definitely agree on the idea of expanding the use case to the
> >>>>>general domain where dmabuf sharing is used. However, what you are
> >>>>>targetting with proposed changes is identical to the core design of
> >>>>>hyper_dmabuf.
> >>>>>
> >>>>>On top of this basic functionalities, hyper_dmabuf has driver level
> >>>>>inter-domain communication, that is needed for dma-buf remote tracking
> >>>>>(no fence forwarding though), event triggering and event handling, extra
> >>>>>meta data exchange and hyper_dmabuf_id that represents grefs
> >>>>>(grefs are shared implicitly on driver level)
> >>>>This really isn't a positive design aspect of hyperdmabuf imo. The core
> >>>>code in xen-zcopy (ignoring the ioctl side, which will be cleaned up) is
> >>>>very simple & clean.
> >>>>
> >>>>If there's a clear need later on we can extend that. But for now xen-zcopy
> >>>>seems to cover the basic use-case needs, so gets the job done.
> >>>>
> >>>>>Also it is designed with frontend (common core framework) + backend
> >>>>>(hyper visor specific comm and memory sharing) structure for portability.
> >>>>>We just can't limit this feature to Xen because we want to use the same
> >>>>>uapis not only for Xen but also other applicable hypervisor, like ACORN.
> >>>>See the discussion around udmabuf and the needs for kvm. I think trying to
> >>>>make an ioctl/uapi that works for multiple hypervisors is misguided - it
> >>>>likely won't work.
> >>>>
> >>>>On top of that the 2nd hypervisor you're aiming to support is ACRN. That's
> >>>>not even upstream yet, nor have I seen any patches proposing to land linux
> >>>>support for ACRN. Since it's not upstream, it doesn't really matter for
> >>>>upstream consideration. I'm doubting that ACRN will use the same grant
> >>>>references as xen, so the same uapi won't work on ACRN as on Xen anyway.
> >>>Yeah, ACRN doesn't have grant-table. Only Xen supports it. But that is why
> >>>hyper_dmabuf has been architectured with the concept of backend.
> >>>If you look at the structure of backend, you will find that
> >>>backend is just a set of standard function calls as shown here:
> >>>
> >>>struct hyper_dmabuf_bknd_ops {
> >>>         /* backend initialization routine (optional) */
> >>>         int (*init)(void);
> >>>
> >>>         /* backend cleanup routine (optional) */
> >>>         int (*cleanup)(void);
> >>>
> >>>         /* retreiving id of current virtual machine */
> >>>         int (*get_vm_id)(void);
> >>>
> >>>         /* get pages shared via hypervisor-specific method */
> >>>         int (*share_pages)(struct page **pages, int vm_id,
> >>>                            int nents, void **refs_info);
> >>>
> >>>         /* make shared pages unshared via hypervisor specific method */
> >>>         int (*unshare_pages)(void **refs_info, int nents);
> >>>
> >>>         /* map remotely shared pages on importer's side via
> >>>          * hypervisor-specific method
> >>>          */
> >>>         struct page ** (*map_shared_pages)(unsigned long ref, int vm_id,
> >>>                                            int nents, void **refs_info);
> >>>
> >>>         /* unmap and free shared pages on importer's side via
> >>>          * hypervisor-specific method
> >>>          */
> >>>         int (*unmap_shared_pages)(void **refs_info, int nents);
> >>>
> >>>         /* initialize communication environment */
> >>>         int (*init_comm_env)(void);
> >>>
> >>>         void (*destroy_comm)(void);
> >>>
> >>>         /* upstream ch setup (receiving and responding) */
> >>>         int (*init_rx_ch)(int vm_id);
> >>>
> >>>         /* downstream ch setup (transmitting and parsing responses) */
> >>>         int (*init_tx_ch)(int vm_id);
> >>>
> >>>         int (*send_req)(int vm_id, struct hyper_dmabuf_req *req, int 
> >>> wait);
> >>>};
> >>>
> >>>All of these can be mapped with any hypervisor specific implementation.
> >>>We designed backend implementation for Xen using grant-table, Xen event
> >>>and ring buffer communication. For ACRN, we have another backend using 
> >>>Virt-IO
> >>>for both memory sharing and communication.
> >>>
> >>>We tried to define this structure of backend to make it general enough (or
> >>>it can be even modified or extended to support more cases.) so that it can
> >>>fit to other hypervisor cases. Only requirements/expectation on the 
> >>>hypervisor
> >>>are page-level memory sharing and inter-domain communication, which I think
> >>>are standard features of modern hypervisor.
> >>>
> >>>And please review common UAPIs that hyper_dmabuf and xen-zcopy supports. 
> >>>They
> >>>are very general. One is getting FD (dmabuf) and get those shared. The 
> >>>other
> >>>is generating dmabuf from global handle (secure handle hiding gref behind 
> >>>it).
> >>>On top of this, hyper_dmabuf has "unshare" and "query" which are also 
> >>>useful
> >>>for any cases.
> >>>
> >>>So I don't know why we wouldn't want to try to make these standard in most 
> >>>of
> >>>hypervisor cases instead of limiting it to certain hypervisor like Xen.
> >>>Frontend-backend structre is optimal for this I think.
> >>>
> >>>>>So I am wondering we can start with this hyper_dmabuf then modify it for
> >>>>>your use-case if needed and polish and fix any glitches if we want to
> >>>>>to use this for all general dma-buf usecases.
> >>>>Imo xen-zcopy is a much more reasonable starting point for upstream, which
> >>>>can then be extended (if really proven to be necessary).
> >>>>
> >>>>>Also, I still have one unresolved question regarding the export/import 
> >>>>>flow
> >>>>>in both of hyper_dmabuf and xen-zcopy.
> >>>>>
> >>>>>@danvet: Would this flow (guest1->import existing dmabuf->share 
> >>>>>underlying
> >>>>>pages->guest2->map shared pages->create/export dmabuf) be acceptable now?
> >>>>I think if you just look at the pages, and make sure you handle the
> >>>>sg_page == NULL case it's ok-ish. It's not great, but mostly it should
> >>>>work. The real trouble with hyperdmabuf was the forwarding of all these
> >>>>calls, instead of just passing around a list of grant references.
> >>>I talked to danvet about this litte bit.
> >>>
> >>>I think there was some misunderstanding on this "forwarding". Exporting
> >>>and importing flow in hyper_dmabuf are basically same as xen-zcopy's. I 
> >>>think
> >>>what made confusion was that importing domain notifies exporting domain 
> >>>when
> >>>there are dmabuf operations (like attach, mapping, detach and release) so 
> >>>that
> >>>exporting domain can track the usage of dmabuf on the importing domain.
> >>>
> >>>I designed this for some basic tracking. We may not need to notify for 
> >>>every
> >>>different activity but if none of them is there, exporting domain can't
> >>>determine if it is ok to unshare the buffer or the originator (like i915)
> >>>can free the object even if it's being accessed in importing domain.
> >>>
> >>>Anyway I really hope we can have enough discussion and resolve all concerns
> >>>before nailing it down.
> >>Let me explain how this works in case of para-virtual display
> >>use-case with xen-zcopy.
> >>
> >>1. There are 4 components in the system:
> >>   - displif protocol [1]
> >>   - xen-front - para-virtual DRM driver running in DomU (Guest) VM
> >>   - backend - user-space application running in Dom0
> >>   - xen-zcopy - DRM (as of now) helper driver running in Dom0
> >>
> >>2. All the communication between domains happens between xen-front and the
> >>backend, so it is possible to implement para-virtual display use-case
> >>without xen-zcopy at all (this is why it is a helper driver), but in this
> >>case
> >>memory copying occurs (this is out of scope for this discussion).
> >>
> >>3. To better understand security issues let's see what use-cases we have:
> >>
> >>3.1 xen-front exports its dma-buf (dumb) to the backend
> >>
> >>In this case there are no security issues at all as Dom0 (backend side)
> >>will use DomU's pages (xen-front side) and Dom0 is a trusted domain, so
> >>we assume it won't hurt DomU. Even if DomU dies nothing bad happens to Dom0.
> >>If DomU misbehaves it can only write to its own pages shared with Dom0, but
> >>still
> >>cannot go beyond that, e.g. it can't access Dom0's memory.
> >>
> >>3.2 Backend exports dma-buf to xen-front
> >>
> >>In this case Dom0 pages are shared with DomU. As before, DomU can only write
> >>to these pages, not any other page from Dom0, so it can be still considered
> >>safe.
> >>But, the following must be considered (highlighted in xen-front's Kernel
> >>documentation):
> >>  - If guest domain dies then pages/grants received from the backend cannot
> >>    be claimed back - think of it as memory lost to Dom0 (won't be used for
> >>any
> >>    other guest)
> >>  - Misbehaving guest may send too many requests to the backend exhausting
> >>    its grant references and memory (consider this from security POV). As 
> >> the
> >>    backend runs in the trusted domain we also assume that it is trusted as
> >>well,
> >>    e.g. must take measures to prevent DDoS attacks.
> >>
> >There is another security issue that this driver itself can cause. Using the
> >grant-reference as is is not very safe because it's easy to guess (counting
> >number probably) and any attackers running on the same importing domain can
> >use these references to map shared pages and access the data. This is why we
> >implemented "hyper_dmabuf_id" that contains 96 bit random number to make it
> >almost impossible to guess.
> Yes, there is something to think about in general, not related
> to dma-buf/zcopy. This is a question to Xen community what they
> see as the right approach here.

IMO, this secure global handle should be taken into consideration because
grefs are just plain references generated from in-kernel functions and
how securely this is delivered and used is up to another driver that exposes
these via uapi. And proper way to protect these and also prevent any "guessed"
references from being used may be exchanging and keeping those in the kernel

> >  All grant references for pages are shared in the
> >driver level. This is another reason for having inter-VM comm.
> >
> >>4. xen-front/backend/xen-zcopy synchronization
> >>
> >>4.1. As I already said in 2) all the inter VM communication happens between
> >>xen-front and the backend, xen-zcopy is NOT involved in that.
> >Yeah, understood but this is also my point. Both hyper_dmabuf and xen-zcopy
> >is a driver that expands dmabuf sharing to inter-VM level. Then shouldn't 
> >this
> >driver itself provide some way to synchronize between two VMs?
> No, because xen-zcopy is a *helper* driver, not more.
> >  I think the
> >assumption behind this is that Xen PV display interface and backend (running
> >on the userspace) are used together with xen-zcopy
> Backend may use xen-zcopy or may not - it depends if you need
> zero copy or not, e.g. it is not a must for the backend
> >but what if an user space
> >just want to use xen-zcopy separately? Since it exposes ioctls, this is
> >possible unless you add some dependency configuration there.
> It is possible, any backend (user-space application) can use xen-zcopy
> Even more, one can extend it to provide kernel side API
> >
> >>When xen-front wants to destroy a display buffer (dumb/dma-buf) it issues a
> >>This call is synchronous, so xen-front expects that backend does free the
> >>buffer pages on return.
> >Does it mean importing domain (dom0 assuming we do domU -> dom0 dmabuf
> >exporting) makes a destory request to the exporting VM?
> No, the requester is always DomU, so "destroy buffer" request
> will always come from DomU
> >  But isn't it
> >the domU to make such decision since it's the owner of buffer.
> See above
> >
> >And what about the other way around? For example, what happens if the
> >originator of buffer (like i915) decides to free the object behind dmabuf?
> For that reason there is ref-counting for dma-buf, e.g.
> if i915 decides to free then the backend (in my case) still holds
> the buffer, thus not allowing it do disappear. Basically, this is
> the backend which creates dma-buf from refs and owns it.

ok, I got it. So the xen-zcopy is staying as importer holding one ref
of dmabuf from i915. I see no problem here then. But actually my concern
is more about between domains (below).

> >Would i915 or exporting side of xen-zcopy know whether dom0 currently
> >uses the dmabuf or not?
> Why do you need this to know (probably I don't understand the use-case).
> I could be obvious here, but if ref-count of the dma-buf is not zero
> it is still exists and used?
> >
> >And again, I think this tracking should be handled in the driver itself
> >implicitly without any userspace involvement if we want to this dmabuf
> >sharing exist as a generic feature.
> Why not allow dma-buf Linux framework do that for you?

yes, between hyper_dmabuf/xen-zcopy and i915 (domU) and between end-consumer
and hyper_dmabuf/xen-zcopy (dom0), standard dma-buf protocols work. What
I am referring to is more about between domains. Let's say you want to
clear up sharing from domU, how does it know if it's safe? Does wait ioctl
handles this remote activities? Possibly this can be done by backend in your
scheme but again, this means there's dependency and another reason xen-zcopy
may not be used safely in general dmabuf sharing cases. I think this
part is guaranteed inside the driver that does export/import dmabuf
to/from other domain.

> >
> >>4.2. Backend, on XENDISPL_OP_DBUF_DESTROY:
> >>   - closes all dumb handles/fd's of the buffer according to [3]
> >>   - issues DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE IOCTL to xen-zcopy to make
> >>sure
> >>     the buffer is freed (think of it as it waits for dma-buf->release
> >>callback)
> >>   - replies to xen-front that the buffer can be destroyed.
> >>This way deletion of the buffer happens synchronously on both Dom0 and DomU
> >>sides. In case if DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE returns with time-out
> >>error
> >>(BTW, wait time is a parameter of this IOCTL), Xen will defer grant
> >>reference
> >>removal and will retry later until those are free.
> >>
> >>Hope this helps understand how buffers are synchronously deleted in case
> >>of xen-zcopy with a single protocol command.
> >>
> >>I think the above logic can also be re-used by the hyper-dmabuf driver with
> >>some additional work:
> >>
> >>1. xen-zcopy can be split into 2 parts and extend:
> >>1.1. Xen gntdev driver [4], [5] to allow creating dma-buf from grefs and
> >>vise versa,
> >>implement "wait" ioctl (wait for dma-buf->release): currently these are
> >>1.2. Xen balloon driver [6] to allow allocating contiguous buffers (not
> >>needed
> >>by current hyper-dmabuf, but is a must for xen-zcopy use-cases)
> >Not sure how to match our use case to xen-zcopy's case but we don't do alloc
> >/free all the time.
> We also don't
> >  Also, dom0 won't make any freeing request to domU since it
> >doesn't own the buffer. It only follows dmabuf protocol as such attach/detach
> >/release,
> Similar here
> >  which are tracked by domU (exporting VM). And for destruction of
> >sharing, we have separate IOCTL for that, which revoke grant references "IF"
> >there is no drivers attached to the dmabuf in dom0. Otherwise, it schedules
> >destruction of sharing until it gets final dmabuf release message from dom0.
> We block instead with 3sec timeout + some other logic
> (out of context now)

Does it mean it's not upon some kind of release signal from dom0?

> >
> >Also, in our usecase, (although we didn't intend to do so) it ends up using
> >3~4 buffers repeately.
> 2-3 in our use-cases
> >This is because DRM in domU (that renders) doesn't
> >allocate more object for EGL image since there is always free objects used
> >before exist in the list. And we actually don't do full-path exporting
> >(extracting pages -> grant-references -> get those shared) all the time.
> >If the same dmabuf is exported already, we just update private message then
> >notifies dom0 (reason for hash tables for keeping exported and importer
> >dmabufs).
> In my case these 2-3 buffers are allocated at start and not freed
> until the end - these are used as frame buffers which are constantly
> flipped. So, in my case there is no much profit in trying to cache
> which adds unneeded complexity (in my use-case, of course).
> If those 3-4 buffers you allocate are the only buffers used you may
> also try going without caching, but this depends on your use-case

I have a question about your use case. So is display manager on dom0
importing 2~3 dedicated dmabufs from domU during the initialization then use
those only or domU keeps exporting buffers to dom0 at every swap but those
happen to be same one used before in normal situation?

This is just my thought and might be limited to our usecase but
Wouldn't the latter be more natural and flexible where client app in
domU renders to its own object then export it to the compositor running on
dom0 at every swap. And meanwhile the hyper_dmabuf/xen-zcopy handles a way
to do this efficiently without any duplication of sharing. With this,
userspace doesn't even have to know about intial preparation or protocol
it needs to follow to share buffers between two domains (e.g. preallocation
of sharable objects)

> >>2. Then hyper-dmabuf uses Xen gntdev driver for Xen specific dma-buf
> >>alloc/free/wait
> >>
> >>3. hyper-dmabuf uses its own protocol between VMs to communicate buffer
> >>creation/deletion and whatever else is needed (fences?).
> >>
> >>To Xen community: please think of dma-buf here as of a buffer representation
> >>mechanism,
> >>e.g. at the end of the day it's just a set of pages.
> >>
> >>Thank you,
> >>Oleksandr
> >>>>-Daniel
> >>>>
> >>>>>Regards,
> >>>>>DW
> >>>>>On Mon, Apr 16, 2018 at 05:33:46PM +0300, Oleksandr Andrushchenko wrote:
> >>>>>>Hello, all!
> >>>>>>
> >>>>>>After discussing xen-zcopy and hyper-dmabuf [1] approaches
> >>>>>>
> >>>>>>it seems that xen-zcopy can be made not depend on DRM core any more
> >>>>>>
> >>>>>>and be dma-buf centric (which it in fact is).
> >>>>>>
> >>>>>>The DRM code was mostly there for dma-buf's FD import/export
> >>>>>>
> >>>>>>with DRM PRIME UAPI and with DRM use-cases in mind, but it comes out 
> >>>>>>that if
> >>>>>>
> >>>>>>the proposed 2 IOCTLs (DRM_XEN_ZCOPY_DUMB_FROM_REFS and
> >>>>>>
> >>>>>>are extended to also provide a file descriptor of the corresponding 
> >>>>>>dma-buf,
> >>>>>>then
> >>>>>>
> >>>>>>PRIME stuff in the driver is not needed anymore.
> >>>>>>
> >>>>>>That being said, xen-zcopy can safely be detached from DRM and moved 
> >>>>>>from
> >>>>>>
> >>>>>>drivers/gpu/drm/xen into drivers/xen/dma-buf-backend(?).
> >>>>>>
> >>>>>>This driver then becomes a universal way to turn any shared buffer 
> >>>>>>between
> >>>>>>Dom0/DomD
> >>>>>>
> >>>>>>and DomU(s) into a dma-buf, e.g. one can create a dma-buf from any grant
> >>>>>>references
> >>>>>>
> >>>>>>or represent a dma-buf as grant-references for export.
> >>>>>>
> >>>>>>This way the driver can be used not only for DRM use-cases, but also for
> >>>>>>other
> >>>>>>
> >>>>>>use-cases which may require zero copying between domains.
> >>>>>>
> >>>>>>For example, the use-cases we are about to work in the nearest future 
> >>>>>>will
> >>>>>>use
> >>>>>>
> >>>>>>V4L, e.g. we plan to support cameras, codecs etc. and all these will 
> >>>>>>benefit
> >>>>>>
> >>>>>>from zero copying much. Potentially, even block/net devices may benefit,
> >>>>>>but this needs some evaluation.
> >>>>>>
> >>>>>>
> >>>>>>I would love to hear comments for authors of the hyper-dmabuf
> >>>>>>
> >>>>>>and Xen community, as well as DRI-Devel and other interested parties.
> >>>>>>
> >>>>>>
> >>>>>>Thank you,
> >>>>>>
> >>>>>>Oleksandr
> >>>>>>
> >>>>>>
> >>>>>>On 03/29/2018 04:19 PM, Oleksandr Andrushchenko wrote:
> >>>>>>>From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> >>>>>>>
> >>>>>>>Hello!
> >>>>>>>
> >>>>>>>When using Xen PV DRM frontend driver then on backend side one will 
> >>>>>>>need
> >>>>>>>to do copying of display buffers' contents (filled by the
> >>>>>>>frontend's user-space) into buffers allocated at the backend side.
> >>>>>>>Taking into account the size of display buffers and frames per seconds
> >>>>>>>it may result in unneeded huge data bus occupation and performance 
> >>>>>>>loss.
> >>>>>>>
> >>>>>>>This helper driver allows implementing zero-copying use-cases
> >>>>>>>when using Xen para-virtualized frontend display driver by
> >>>>>>>implementing a DRM/KMS helper driver running on backend's side.
> >>>>>>>It utilizes PRIME buffers API to share frontend's buffers with
> >>>>>>>physical device drivers on backend's side:
> >>>>>>>
> >>>>>>>  - a dumb buffer created on backend's side can be shared
> >>>>>>>    with the Xen PV frontend driver, so it directly writes
> >>>>>>>    into backend's domain memory (into the buffer exported from
> >>>>>>>    DRM/KMS driver of a physical display device)
> >>>>>>>  - a dumb buffer allocated by the frontend can be imported
> >>>>>>>    into physical device DRM/KMS driver, thus allowing to
> >>>>>>>    achieve no copying as well
> >>>>>>>
> >>>>>>>For that reason number of IOCTLs are introduced:
> >>>>>>>     This will create a DRM dumb buffer from grant references provided
> >>>>>>>     by the frontend
> >>>>>>>    This will grant references to a dumb/display buffer's memory 
> >>>>>>> provided
> >>>>>>>    by the backend
> >>>>>>>    This will block until the dumb buffer with the wait handle provided
> >>>>>>>    be freed
> >>>>>>>
> >>>>>>>With this helper driver I was able to drop CPU usage from 17% to 3%
> >>>>>>>on Renesas R-Car M3 board.
> >>>>>>>
> >>>>>>>This was tested with Renesas' Wayland-KMS and backend running as DRM 
> >>>>>>>master.
> >>>>>>>
> >>>>>>>Thank you,
> >>>>>>>Oleksandr
> >>>>>>>
> >>>>>>>Oleksandr Andrushchenko (1):
> >>>>>>>   drm/xen-zcopy: Add Xen zero-copy helper DRM driver
> >>>>>>>
> >>>>>>>  Documentation/gpu/drivers.rst               |   1 +
> >>>>>>>  Documentation/gpu/xen-zcopy.rst             |  32 +
> >>>>>>>  drivers/gpu/drm/xen/Kconfig                 |  25 +
> >>>>>>>  drivers/gpu/drm/xen/Makefile                |   5 +
> >>>>>>>  drivers/gpu/drm/xen/xen_drm_zcopy.c         | 880 
> >>>>>>> ++++++++++++++++++++++++++++
> >>>>>>>  drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c | 154 +++++
> >>>>>>>  drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h |  38 ++
> >>>>>>>  include/uapi/drm/xen_zcopy_drm.h            | 129 ++++
> >>>>>>>  8 files changed, 1264 insertions(+)
> >>>>>>>  create mode 100644 Documentation/gpu/xen-zcopy.rst
> >>>>>>>  create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy.c
> >>>>>>>  create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.c
> >>>>>>>  create mode 100644 drivers/gpu/drm/xen/xen_drm_zcopy_balloon.h
> >>>>>>>  create mode 100644 include/uapi/drm/xen_zcopy_drm.h
> >>>>>>>
> >>>>>>[1]
> >>>>>>https://lists.xenproject.org/archives/html/xen-devel/2018-02/msg01202.html
> >>>>>_______________________________________________
> >>>>>dri-devel mailing list
> >>>>>dri-devel@xxxxxxxxxxxxxxxxxxxxx
> >>>>>https://lists.freedesktop.org/mailman/listinfo/dri-devel
> >>>>-- 
> >>>>Daniel Vetter
> >>>>Software Engineer, Intel Corporation
> >>>>http://blog.ffwll.ch
> >>[1] 
> >>https://elixir.bootlin.com/linux/v4.17-rc1/source/include/xen/interface/io/displif.h
> >>[2] 
> >>https://elixir.bootlin.com/linux/v4.17-rc1/source/include/xen/interface/io/displif.h#L539
> >>[3] 
> >>https://elixir.bootlin.com/linux/v4.17-rc1/source/drivers/gpu/drm/drm_prime.c#L39
> >>[4] https://elixir.bootlin.com/linux/v4.17-rc1/source/drivers/xen/gntdev.c
> >>[5]
> >>https://elixir.bootlin.com/linux/v4.17-rc1/source/include/uapi/xen/gntdev.h
> >>[6] https://elixir.bootlin.com/linux/v4.17-rc1/source/drivers/xen/balloon.c

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