Re: [RfC PATCH] Add udmabuf misc device

[Oleksandr Andrushchenko <andr2000@gmail.com>]

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