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 11:07:21 +0300 |
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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: >>>>>>>> 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. > 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
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