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

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

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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