Re: [Qemu-devel] Using virtio for inter-VM communication

[Rusty Russell]

Jan Kiszka <address@hidden> writes:
> On 2014-06-12 04:27, Rusty Russell wrote:
>> Henning Schild <address@hidden> writes:
>> It was also never implemented, and remains a thought experiment.
>> However, implementing it in lguest should be fairly easy.
>
> The reason why a trusted helper, i.e. additional logic in the
> hypervisor, is not our favorite solution is that we'd like to keep the
> hypervisor as small as possible. I wouldn't exclude such an approach
> categorically, but we have to weigh the costs (lines of code, additional
> hypervisor interface) carefully against the gain (existing
> specifications and guest driver infrastructure).

Reasonable, but I think you'll find it is about the minimal
implementation in practice.  Unfortunately, I don't have time during the
next 6 months to implement it myself :(

> Back to VIRTIO_F_RING_SHMEM_ADDR (which you once brought up in an MCA
> working group discussion): What speaks against introducing an
> alternative encoding of addresses inside virtio data structures? The
> idea of this flag was to replace guest-physical addresses with offsets
> into a shared memory region associated with or part of a virtio
> device.

We would also need a way of defining the shared memory region.  But
that's not the problem.  If such a feature is not accepted by the guest?
How to you fall back?

We don't add features which unmake the standard.

> That would preserve zero-copy capabilities (as long as you can work
> against the shared mem directly, e.g. doing DMA from a physical NIC or
> storage device into it) and keep the hypervisor out of the loop.

This seems ill thought out.  How will you program a NIC via the virtio
protocol without a hypervisor?  And how will you make it safe?  You'll
need an IOMMU.  But if you have an IOMMU you don't need shared memory.

> Is it
> too invasive to existing infrastructure or does it have some other pitfalls?

You'll have to convince every vendor to implement your addition to the
standard.  Which is easier than inventing a completely new system, but
it's not quite virtio.

Cheers,
Rusty.