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

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

On 04/24/2018 12:08 PM, Juergen Gross wrote:
> On 24/04/18 11:03, Oleksandr Andrushchenko wrote:
>> On 04/24/2018 11:40 AM, Juergen Gross wrote:
>>> On 24/04/18 10:07, Oleksandr Andrushchenko wrote:
>>>> 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
>>> Would this program use libgnttab in parallel?
>> In case of the display backend - yes, for shared rings,
>> extracting grefs from displif protocol it uses gntdev via
>> helper library [1]
>>>    If yes how would the two
>>> usage paths be combined (same applies to the separate driver, btw)? The
>>> gntdev driver manages resources per file descriptor and libgnttab is
>>> hiding the file descriptor it is using for a connection.
>> Ah, at the moment the UAPI was not used in parallel as there were
>> 2 drivers for that: gntdev + xen-zcopy with different UAPIs.
>> But now, if we extend gntdev with the new API then you are rigth:
>> either libgnttab needs to be extended or that new part of the
>> gntdev UAPI needs to be open-coded by the backend
>>>    Or would the
>>> user program use only the new driver for communicating with the gntdev
>>> driver? In this case it might be an option to extend the gntdev driver
>>> to present a new device (e.g. "gntdmadev") for that purpose.
>> No, it seems that libgnttab and this new driver's UAPI will be used
>> in parallel
>>>>> 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)
>>> I think in the end this really depends on how the complete solution
>>> will look like. gntdev is a special wrapper for the gnttab driver.
>>> In case the new dma-buf driver needs to use parts of gntdev I'd rather
>>> have a new driver above gnttab ("gntuser"?) used by gntdev and dma-buf.
>> The new driver doesn't use gntdev's existing API, but extends it,
>> e.g. by adding new ways to export/import grefs for a dma-buf and
>> manage dma-buf's kernel ops. Thus, gntdev, which already provides
>> UAPI, seems to be a good candidate for such an extension
> So this would mean you need a modification of libgnttab, right? This is
> something the Xen tools maintainers need to decide. In case they don't
> object extending the gntdev driver would be the natural thing to do.
Wei is already in the thread, adding Ian
>
> Juergen