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    SubjectRe: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver
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    On 04/24/2018 10:51 AM, Juergen Gross wrote:
    > On 24/04/18 07:43, Oleksandr Andrushchenko wrote:
    >> On 04/24/2018 01:41 AM, Boris Ostrovsky wrote:
    >>> On 04/23/2018 08:10 AM, Oleksandr Andrushchenko wrote:
    >>>> On 04/23/2018 02:52 PM, Wei Liu wrote:
    >>>>> On Fri, Apr 20, 2018 at 02:25:20PM +0300, Oleksandr Andrushchenko
    >>>>> wrote:
    >>>>>>>>       the gntdev.
    >>>>>>>>
    >>>>>>>> I think this is generic enough that it could be implemented by a
    >>>>>>>> device not tied to Xen. AFAICT the hyper_dma guys also wanted
    >>>>>>>> something similar to this.
    >>>>>>> You can't just wrap random userspace memory into a dma-buf. We've
    >>>>>>> just had
    >>>>>>> this discussion with kvm/qemu folks, who proposed just that, and
    >>>>>>> after a
    >>>>>>> bit of discussion they'll now try to have a driver which just wraps a
    >>>>>>> memfd into a dma-buf.
    >>>>>> So, we have to decide either we introduce a new driver
    >>>>>> (say, under drivers/xen/xen-dma-buf) or extend the existing
    >>>>>> gntdev/balloon to support dma-buf use-cases.
    >>>>>>
    >>>>>> Can anybody from Xen community express their preference here?
    >>>>>>
    >>>>> Oleksandr talked to me on IRC about this, he said a few IOCTLs need to
    >>>>> be added to either existing drivers or a new driver.
    >>>>>
    >>>>> I went through this thread twice and skimmed through the relevant
    >>>>> documents, but I couldn't see any obvious pros and cons for either
    >>>>> approach. So I don't really have an opinion on this.
    >>>>>
    >>>>> But, assuming if implemented in existing drivers, those IOCTLs need to
    >>>>> be added to different drivers, which means userspace program needs to
    >>>>> write more code and get more handles, it would be slightly better to
    >>>>> implement a new driver from that perspective.
    >>>> If gntdev/balloon extension is still considered:
    >>>>
    >>>> All the IOCTLs will be in gntdev driver (in current xen-zcopy
    >>>> terminology):
    >>>>   - DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS
    >>>>   - DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
    >>>>   - DRM_IOCTL_XEN_ZCOPY_DUMB_WAIT_FREE
    >>>>
    >>>> Balloon driver extension, which is needed for contiguous/DMA
    >>>> buffers, will be to provide new *kernel API*, no UAPI is needed.
    >>>>
    >>> So I am obviously a bit late to this thread, but why do you need to add
    >>> new ioctls to gntdev and balloon? Doesn't this driver manage to do what
    >>> you want without any extensions?
    >> 1. I only (may) need to add IOCTLs to gntdev
    >> 2. balloon driver needs to be extended, so it can allocate
    >> contiguous (DMA) memory, not IOCTLs/UAPI here, all lives
    >> in the kernel.
    >> 3. The reason I need to extend gnttab with new IOCTLs is to
    >> provide new functionality to create a dma-buf from grant references
    >> and to produce grant references for a dma-buf. This is what I have as UAPI
    >> description for xen-zcopy driver:
    >>
    >> 1. DRM_IOCTL_XEN_ZCOPY_DUMB_FROM_REFS
    >> This will create a DRM dumb buffer from grant references provided
    >> by the frontend. The intended usage is:
    >>   - Frontend
    >>     - creates a dumb/display buffer and allocates memory
    >>     - grants foreign access to the buffer pages
    >>     - passes granted references to the backend
    >>   - Backend
    >>     - issues DRM_XEN_ZCOPY_DUMB_FROM_REFS ioctl to map
    >>       granted references and create a dumb buffer
    >>     - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD
    >>     - requests real HW driver/consumer to import the PRIME buffer with
    >>       DRM_IOCTL_PRIME_FD_TO_HANDLE
    >>     - uses handle returned by the real HW driver
    >>   - at the end:
    >>     o closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE
    >>     o closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE
    >>     o closes file descriptor of the exported buffer
    >>
    >> 2. DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
    >> This will grant references to a dumb/display buffer's memory provided by
    >> the
    >> backend. The intended usage is:
    >>   - Frontend
    >>     - requests backend to allocate dumb/display buffer and grant references
    >>       to its pages
    >>   - Backend
    >>     - requests real HW driver to create a dumb with
    >> DRM_IOCTL_MODE_CREATE_DUMB
    >>     - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD
    >>     - requests zero-copy driver to import the PRIME buffer with
    >>       DRM_IOCTL_PRIME_FD_TO_HANDLE
    >>     - issues DRM_XEN_ZCOPY_DUMB_TO_REFS ioctl to
    >>       grant references to the buffer's memory.
    >>     - passes grant references to the frontend
    >>  - at the end:
    >>     - closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE
    >>     - closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE
    >>     - closes file descriptor of the imported buffer
    >>
    >> 3. DRM_XEN_ZCOPY_DUMB_WAIT_FREE
    >> This will block until the dumb buffer with the wait handle provided be
    >> freed:
    >> this is needed for synchronization between frontend and backend in case
    >> frontend provides grant references of the buffer via
    >> DRM_XEN_ZCOPY_DUMB_FROM_REFS IOCTL and which must be released before
    >> backend replies with XENDISPL_OP_DBUF_DESTROY response.
    >> wait_handle must be the same value returned while calling
    >> DRM_XEN_ZCOPY_DUMB_FROM_REFS IOCTL.
    >>
    >> So, as you can see the above functionality is not covered by the
    >> existing UAPI
    >> of the gntdev driver.
    >> Now, if we change dumb -> dma-buf and remove DRM code (which is only a
    >> wrapper
    >> here on top of dma-buf) we get new driver for dma-buf for Xen.
    >>
    >> This is why I have 2 options here: either create a dedicated driver for
    >> this
    >> (e.g. re-work xen-zcopy to be DRM independent and put it under
    >> drivers/xen/xen-dma-buf, for example) or extend the existing gntdev driver
    >> with the above UAPI + make changes to the balloon driver to provide kernel
    >> API for DMA buffer allocations.
    > Which user component would use the new ioctls?
    It is currently used by the display backend [1] and will
    probably be used by the hyper-dmabuf frontend/backend
    (Dongwon from Intel can provide more info on this).
    >
    > I'm asking because I'm not very fond of adding more linux specific
    > functions to libgnttab which are not related to a specific Xen version,
    > but to a kernel version.
    Hm, I was not thinking about this UAPI to be added to libgnttab.
    It seems it can be used directly w/o wrappers in user-space
    >
    > So doing this in a separate driver seems to be the better option in
    > this regard.
    Well, from maintenance POV it is easier for me to have it all in
    a separate driver as all dma-buf related functionality will
    reside at one place. This also means that no changes to existing
    drivers will be needed (if it is ok to have ballooning in/out
    code for DMA buffers (allocated with dma_alloc_xxx) not in the balloon
    driver)
    >
    > Juergen
    [1] https://github.com/xen-troops/displ_be

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