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Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device

From: Oleksandr Andrushchenko
Subject: Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device
Date: Fri, 6 Apr 2018 12:34:45 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 04/06/2018 12:07 PM, Gerd Hoffmann wrote:

   * The general interface should be able to express sharing from any
     guest:guest, not just guest:host.  Arbitrary G:G sharing might be
     something some hypervisors simply aren't able to support, but the
     userspace API itself shouldn't make assumptions or restrict that.  I
     think ideally the sharing API would include some kind of
     query_targets interface that would return a list of VM's that your
     current OS is allowed to share with; that list would be depend on the
     policy established by the system integrator, but obviously wouldn't
     include targets that the hypervisor itself wouldn't be capable of
Can you give a use-case for this? I mean that the system integrator
is the one who defines which guests/hosts talk to each other,
but querying means that it is possible that VMs have some sort
of discovery mechanism, so they can decide on their own whom
to connect to.
Note that vsock (created by vmware, these days also has a virtio
transport for kvm) started with support for both guest <=> host and
guest <=> guest support.  But later on guest <=> guest was dropped.
As far I know the reasons where (a) lack of use cases and (b) security.

So, I likewise would know more details on the use cases you have in mind
here.  Unless we have a compelling use case here I'd suggest to drop the
guest <=> guest requirement as it makes the whole thing alot more
This is exactly the use-case we have: in our setup Dom0 doesn't
own any HW at all and all the HW is passed into a dedicated
driver domain (DomD) which is still a guest domain.
Then, buffers are shared between two guests, for example,
DomD and DomA (Android guest)
   * The sharing API could be used to share multiple kinds of content in a
     single system.  The sharing sink driver running in the content
     producer's VM should accept some additional metadata that will be
     passed over to the target VM as well.
Not sure this should be part of hyper-dmabuf.  A dma-buf is nothing but
a block of data, period.  Therefore protocols with dma-buf support
(wayland for example) typically already send over metadata describing
the content, so duplicating that in hyper-dmabuf looks pointless.

1. We are targeting ARM and one of the major requirements for the buffer
sharing is the ability to allocate physically contiguous buffers, which gets
even more complicated for systems not backed with an IOMMU. So, for some
use-cases it is enough to make the buffers contiguous in terms of IPA and
sometimes those need to be contiguous in terms of PA.
Which pretty much implies the host must to the allocation.

2. For Xen we would love to see UAPI to create a dma-buf from grant
references provided, so we can use this generic solution to implement
zero-copying without breaking the existing Xen protocols. This can
probably be extended to other hypervizors as well.
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.
Please see above for our setup with DomD and Dom0 being
a generic ARMv8 domain, no HW
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
This is what it does

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 ...

Beside that there is the problem that the udmabuf idea has its own share
of issues, for example the fork() issue pointed out by Christian
K├Ânig[1].  So I still need to find something which can work for kvm ...


[1] https://www.spinics.net/lists/dri-devel/msg169442.html
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