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

[Oleksandr Andrushchenko]

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:
> > > > > > > >  ÂÂÂÂÂ 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):
> > > >  ÂÂ- DRM_ICOTL_XEN_ZCOPY_DUMB_FROM_REFS
> > > >  ÂÂ- DRM_IOCTL_XEN_ZCOPY_DUMB_TO_REFS
> > > >  ÂÂ- 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:
> >  Â - Frontend
> >  ÂÂÂ - creates a dumb/display buffer and allocates memory
> >  ÂÂÂ - grants foreign access to the buffer pages
> >  ÂÂÂ - passes granted references to the backend
> >  Â - Backend
> >  ÂÂÂ - issues DRM_XEN_ZCOPY_DUMB_FROM_REFS ioctl to map
> >  ÂÂÂÂÂ granted references and create a dumb buffer
> >  ÂÂÂ - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD
> >  ÂÂÂ - requests real HW driver/consumer to import the PRIME buffer with
> >  ÂÂÂÂÂ DRM_IOCTL_PRIME_FD_TO_HANDLE
> >  ÂÂÂ - uses handle returned by the real HW driver
> >  Â - at the end:
> >  ÂÂÂ o closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE
> >  ÂÂÂ o closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE
> >  ÂÂÂ 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:
> >  Â - Frontend
> >  ÂÂÂ - requests backend to allocate dumb/display buffer and grant references
> >  ÂÂÂÂÂ to its pages
> >  Â - Backend
> >  ÂÂÂ - requests real HW driver to create a dumb with
> > DRM_IOCTL_MODE_CREATE_DUMB
> >  ÂÂÂ - requests handle to fd conversion via DRM_IOCTL_PRIME_HANDLE_TO_FD
> >  ÂÂÂ - requests zero-copy driver to import the PRIME buffer with
> >  ÂÂÂÂÂ DRM_IOCTL_PRIME_FD_TO_HANDLE
> >  ÂÂÂ - issues DRM_XEN_ZCOPY_DUMB_TO_REFS ioctl to
> >  ÂÂÂÂÂ grant references to the buffer's memory.
> >  ÂÂÂ - passes grant references to the frontend
> >  Â- at the end:
> >  ÂÂÂ - closes zero-copy driver's handle with DRM_IOCTL_GEM_CLOSE
> >  ÂÂÂ - closes real HW driver's handle with DRM_IOCTL_GEM_CLOSE
> >  ÂÂÂ - 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