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

Oleksandr Andrushchenko
2018-04-24T10:14:43Z

On 04/24/2018 01:01 PM, Wei Liu wrote:
> On Tue, Apr 24, 2018 at 11:08:41AM +0200, 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.
>>
> That should be fine. Most of libgnttab does is to wrap existing kernel
> interfaces and expose them sensibly to user space programs. If gnttab
> device is extended, libgnttab should be extended accordingly. If a new
> device is created, a new library should be added. Either way there will
> be new toolstack code involved, which is not a problem in general.
Great, so finally I see the following approach to have generic
dma-buf use-cases support for Xen (which can be used for many purposes,
e.g. GPU/DRM buffer sharing, V4L, hyper-dmabuf etc.):

1. Extend Linux gntdev driver to support 3 new IOCTLs discussed previously
2. Extend libgnttab to provide UAPI for those - Linux only as dma-buf
is a Linux thing
3. Extend kernel API of the Linux balloon driver to allow dma_alloc_xxx way
of memory allocations

If the above looks ok, then I can start prototyping, so we can discuss
implementation details

Dongwong - could you please comment on all this if it fits your use-cases
(I do believe it does)?
> Wei.
>
>> Juergen
Thank you,
Oleksandr