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

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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.

So, this is where I need to understand Xen community's preference, so the
implementation is not questioned later on.

> -boris
Thank you,
Oleksandr
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