Messages in this thread | | | Subject | Re: [Xen-devel] [PATCH 0/1] drm/xen-zcopy: Add Xen zero-copy helper DRM driver | From | Oleksandr Andrushchenko <> | Date | Tue, 24 Apr 2018 08:43:37 +0300 |
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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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