[virtio-dev] Zerocopy VM-to-VM networking using virtio-net

Jan Kiszka
Mon, 27 Apr 2015 12:35:19 -0000

Am 2015-04-27 um 12:17 schrieb Stefan Hajnoczi:
> On Sun, Apr 26, 2015 at 2:24 PM, Luke Gorrie <luke at snabb.co>
wrote:
>> On 24 April 2015 at 15:22, Stefan Hajnoczi <stefanha at
gmail.com> wrote:
>>>
>>> The motivation for making VM-to-VM fast is that while software
>>> switches on the host are efficient today (thanks to vhost-user),
there
>>> is no efficient solution if the software switch is a VM.
>>
>>
>> I see. This sounds like a noble goal indeed. I would love to run the
>> software switch as just another VM in the long term. It would make it
much
>> easier for the various software switches to coexist in the world.
>>
>> The main technical risk I see in this proposal is that eliminating the
>> memory copies might not have the desired effect. I might be tempted to
keep
>> the copies but prevent the kernel from having to inspect the vrings
(more
>> like vhost-user). But that is just a hunch and I suppose the first step
>> would be a prototype to check the performance anyway.
>>
>> For what it is worth here is my view of networking performance on x86
in the
>> Haswell+ era:
>> https://groups.google.com/forum/#!topic/snabb-devel/aez4pEnd4ow
>
> Thanks.
>
> I've been thinking about how to eliminate the VM <-> host
<-> VM
> switching and instead achieve just VM <-> VM.
>
> The holy grail of VM-to-VM networking is an exitless I/O path.  In
> other words, packets can be transferred between VMs without any
> vmexits (this requires a polling driver).
>
> Here is how it works.  QEMU gets "-device vhost-user" so that a
VM can
> act as the vhost-user server:
>
> VM1 (virtio-net guest driver) <-> VM2 (vhost-user device)
>
> VM1 has a regular virtio-net PCI device.  VM2 has a vhost-user device
> and plays the host role instead of the normal virtio-net guest driver
> role.
>
> The ugly thing about this is that VM2 needs to map all of VM1's guest
> RAM so it can access the vrings and packet data.  The solution to this
> is something like the Shared Buffers BAR but this time it contains not
> just the packet data but also the vring, let's call it the Shared
> Virtqueues BAR.
>
> The Shared Virtqueues BAR eliminates the need for vhost-net on the
> host because VM1 and VM2 communicate directly using virtqueue notify
> or polling vring memory.  Virtqueue notify works by connecting an
> eventfd as ioeventfd in VM1 and irqfd in VM2.  And VM2 would also have
> an ioeventfd that is irqfd for VM1 to signal completions.

We had such a discussion before:
http://thread.gmane.org/gmane.comp.emulators.kvm.devel/123014/focus=279658

Would be great to get this ball rolling again.

Jan

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