Messages in this thread | | | Subject | Re: [RfC PATCH] Add udmabuf misc device | From | Oleksandr Andrushchenko <> | Date | Fri, 6 Apr 2018 14:17:04 +0300 |
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On 04/06/2018 12:07 PM, Gerd Hoffmann wrote: > I'm not sure we can create something which works on both kvm and xen. > The memory management model is quite different ... > > > On xen the hypervisor manages all memory. Guests can allow other guests > to access specific pages (using grant tables). In theory any guest <=> > guest communication is possible. In practice is mostly guest <=> dom0 > because guests access their virtual hardware that way. dom0 is the > priviledged guest which owns any hardware not managed by xen itself. > > Xen guests can ask the hypervisor to update the mapping of guest > physical pages. They can ballon down (unmap and free pages). They can > ballon up (ask the hypervisor to map fresh pages). They can map pages > exported by other guests using grant tables. xen-zcopy makes heavy use > of this. It balloons down, to make room in the guest physical address > space, then goes map the exported pages there, finally composes a > dma-buf. > > > On kvm qemu manages all guest memory. qemu also has all guest memory > mapped, so a grant-table like mechanism isn't needed to implement > virtual devices. qemu can decide how it backs memory for the guest. > qemu propagates the guest memory map to the kvm driver in the linux > kernel. kvm guests have some control over the guest memory map, for > example they can map pci bars wherever they want in their guest physical > address space by programming the base registers accordingly, but unlike > xen guests they can't ask the host to remap individual pages. > > Due to qemu having all guest memory mapped virtual devices are typically > designed to have the guest allocate resources, then notify the host > where they are located. This is where the udmabuf idea comes from: > Guest tells the host (qemu) where the gem object is, and qemu then can > create a dmabuf backed by those pages to pass it on to other processes > such as the wayland display server. Possibly even without the guest > explicitly asking for it, i.e. export the framebuffer placed by the > guest in the (virtual) vga pci memory bar as dma-buf. And I can imagine > that this is useful outsize virtualization too. > > > 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? And what about hyper-dmabuf?
Thank you, Oleksandr
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