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Re: [Xen-users] How to set up trivial shared-memory buffer between VMs (on ARM) ?

On Thu, 2015-10-01 at 23:20 +0000, Chris (Christopher) Brand wrote:
> Hi,
> I want to set up a simple shared-memory buffer between two VMs on my ARM
> system.
> I was looking at the passthrough stuff, hoping that that might work, but
> it does seem tightly coupled to the use of an IOMMU, which my system
> doesnât have. (
> https://events.linuxfoundation.org/sites/events/files/slides/talk_5.pdf w
> as very helpful for exploring this path, BTW). Itâs fairly trivial to
> move a block of RAM from the memory node to âdeviceâ node in the
> devicetree for Xen, which makes it available to Dom0, but how can I then
> share it with another VM ?

IOMMU's etc are certainly the wrong direction to be looking. The usual way
to do this in a Xen system is for one end to allocate some memory and use
the grant table to expose it to its peer, with the reference usually being
communicated via xenstore.

libvchan is a userspace library which takes care of some of the grunt work
on both client and server end, or alternatively there are some frameworks
in the Linux kernel which can be used to help write drivers (checkout e.g.
either the net or blk front and back for example, unfortunately the in
-kernel ones tend to be for more complex cases).

> For context, I have an existing Linux kernel driver that I want to try
> out under Xen. I suspect that the best way to do so is to migrate it to
> use virtio, but for now I just need to get it running as quickly as
> possible, so Iâd like to minimize the changes needed.

The existing driver should Just Work in dom0, so I suppose you are wanting
to expose the device to a domU (which I suppose is why you were thinking of
doing passthrough). In which case some sort of PV protocol to expose the
functionality is the way to go.

Depending on the type of device you may find a higher level PV abstraction
is better to work with than exposing something "device like".

> Thanks,
> Chris
> P.S. Please move the devel list if thatâs more appropriate
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