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

    -boris

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