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

From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [RfC PATCH] Add udmabuf misc device
Date: Fri, 6 Apr 2018 11:07:47 +0200
User-agent: NeoMutt/20180323


> >   * 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
> >     handling.

> 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

> >   * 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.

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

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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