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 12:03:24 +0300 |
| |
On 04/24/2018 11:40 AM, Juergen Gross wrote: > On 24/04/18 10:07, Oleksandr Andrushchenko wrote: >> 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 > Would this program use libgnttab in parallel? In case of the display backend - yes, for shared rings, extracting grefs from displif protocol it uses gntdev via helper library [1] > If yes how would the two > usage paths be combined (same applies to the separate driver, btw)? The > gntdev driver manages resources per file descriptor and libgnttab is > hiding the file descriptor it is using for a connection. Ah, at the moment the UAPI was not used in parallel as there were 2 drivers for that: gntdev + xen-zcopy with different UAPIs. But now, if we extend gntdev with the new API then you are rigth: either libgnttab needs to be extended or that new part of the gntdev UAPI needs to be open-coded by the backend > Or would the > user program use only the new driver for communicating with the gntdev > driver? In this case it might be an option to extend the gntdev driver > to present a new device (e.g. "gntdmadev") for that purpose. No, it seems that libgnttab and this new driver's UAPI will be used in parallel >>> 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) > I think in the end this really depends on how the complete solution > will look like. gntdev is a special wrapper for the gnttab driver. > In case the new dma-buf driver needs to use parts of gntdev I'd rather > have a new driver above gnttab ("gntuser"?) used by gntdev and dma-buf. The new driver doesn't use gntdev's existing API, but extends it, e.g. by adding new ways to export/import grefs for a dma-buf and manage dma-buf's kernel ops. Thus, gntdev, which already provides UAPI, seems to be a good candidate for such an extension > > Juergen [1] https://github.com/xen-troops/libxenbe
| |