On Fri, Apr 06, 2018 at 03:36:03PM +0300, Oleksandr Andrushchenko wrote: > On 04/06/2018 02:57 PM, Gerd Hoffmann wrote: > > Hi, > > > >>>I fail to see any common ground for xen-zcopy and udmabuf ... > >>Does the above mean you can assume that xen-zcopy and udmabuf > >>can co-exist as two different solutions? > >Well, udmabuf route isn't fully clear yet, but yes. > > > >See also gvt (intel vgpu), where the hypervisor interface is abstracted > >away into a separate kernel modules even though most of the actual vgpu > >emulation code is common. > Thank you for your input, I'm just trying to figure out > which of the three z-copy solutions intersect and how much > >>And what about hyper-dmabuf?
xen z-copy solution is pretty similar fundamentally to hyper_dmabuf in terms of these core sharing feature: 1. the sharing process - import prime/dmabuf from the producer -> extract underlying pages and get those shared -> return references for shared pages 2. the page sharing mechanism - it uses Xen-grant-table. And to give you a quick summary of differences as far as I understand between two implementations (please correct me if I am wrong, Oleksandr.) 1. xen-zcopy is DRM specific - can import only DRM prime buffer while hyper_dmabuf can export any dmabuf regardless of originator 2. xen-zcopy doesn't seem to have dma-buf synchronization between two VMs while (as danvet called it as remote dmabuf api sharing) hyper_dmabuf sends out synchronization message to the exporting VM for synchronization. 3. 1-level references - when using grant-table for sharing pages, there will be same # of refs (each 8 byte) as # of shared pages, which is passed to the userspace to be shared with importing VM in case of xen-zcopy. Compared to this, hyper_dmabuf does multiple level addressing to generate only one reference id that represents all shared pages. 4. inter VM messaging (hype_dmabuf only) - hyper_dmabuf has inter-vm msg communication defined for dmabuf synchronization and private data (meta info that Matt Roper mentioned) exchange. 5. driver-to-driver notification (hyper_dmabuf only) - importing VM gets notified when newdmabuf is exported from other VM - uevent can be optionally generated when this happens. 6. structure - hyper_dmabuf is targetting to provide a generic solution for inter-domain dmabuf sharing for most hypervisors, which is why it has two layers as mattrope mentioned, front-end that contains standard API and backend that is specific to hypervisor. > >No idea, didn't look at it in detail. > > > >Looks pretty complex from a distant view. Maybe because it tries to > >build a communication framework using dma-bufs instead of a simple > >dma-buf passing mechanism. we started with simple dma-buf sharing but realized there are many things we need to consider in real use-case, so we added communication , notification and dma-buf synchronization then re-structured it to front-end and back-end (this made things more compicated..) since Xen was not our only target. Also, we thought passing the reference for the buffer (hyper_dmabuf_id) is not secure so added uvent mechanism later. > Yes, I am looking at it now, trying to figure out the full story > and its implementation. BTW, Intel guys were about to share some > test application for hyper-dmabuf, maybe I have missed one. > It could probably better explain the use-cases and the complexity > they have in hyper-dmabuf. One example is actually in github. If you want take a look at it, please visit: https://github.com/downor/linux_hyper_dmabuf_test/tree/xen/simple_export > > > >Like xen-zcopy it seems to depend on the idea that the hypervisor > >manages all memory it is easy for guests to share pages with the help of > >the hypervisor. > So, for xen-zcopy we were not trying to make it generic, > it just solves display (dumb) zero-copying use-cases for Xen. > We implemented it as a DRM helper driver because we can't see any > other use-cases as of now. > For example, we also have Xen para-virtualized sound driver, but > its buffer memory usage is not comparable to what display wants > and it works somewhat differently (e.g. there is no "frame done" > event, so one can't tell when the sound buffer can be "flipped"). > At the same time, we do not use virtio-gpu, so this could probably > be one more candidate for shared dma-bufs some day. > > Which simply isn't the case on kvm. > > > >hyper-dmabuf and xen-zcopy could maybe share code, or hyper-dmabuf build > >on top of xen-zcopy. > Hm, I can imagine that: xen-zcopy could be a library code for hyper-dmabuf > in terms of implementing all that page sharing fun in multiple directions, > e.g. Host->Guest, Guest->Host, Guest<->Guest. > But I'll let Matt and Dongwon to comment on that. I think we can definitely collaborate. Especially, maybe we are using some outdated sharing mechanism/grant-table mechanism in our Xen backend (thanks for bringing that up Oleksandr). However, the question is once we collaborate somehow, can xen-zcopy's usecase use the standard API that hyper_dmabuf provides? I don't think we need different IOCTLs that do the same in the final solution. > > > > >cheers, > > Gerd > > > Thank you, > Oleksandr > > P.S. Sorry for making your original mail thread to discuss things much > broader than your RFC... >