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

On 04/24/2018 02:54 PM, Daniel Vetter wrote:
On Mon, Apr 23, 2018 at 03:10:35PM +0300, 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):
I was lazy to change dumb to dma-buf, so put this notice ;)
s/DUMB/DMA_BUF/ please. This is generic dma-buf, it has nothing to do with
the dumb scanout buffer support in the drm/gfx subsystem. This here can be
used for any zcopy sharing among guests (as long as your endpoints
understands dma-buf, which most relevant drivers do).
Of course, please see above

Balloon driver extension, which is needed for contiguous/DMA
buffers, will be to provide new *kernel API*, no UAPI is needed.

Thank you,
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