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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 27, 2015 at 02:35:19PM +0200, Jan Kiszka wrote:
> Am 2015-04-27 um 12:17 schrieb Stefan Hajnoczi:
> > On Sun, Apr 26, 2015 at 2:24 PM, Luke Gorrie <luke@snabb.co> wrote:
> >> On 24 April 2015 at 15:22, Stefan Hajnoczi <stefanha@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

I think fundamentally, reducing the stress on the host scheduler
can give a bigger gain than zero copy.

But if I was to implement this, I wouldn't start with the funky virtio
BAR thing.

Start by enabling DPDK vhost-port within guest as-is.
To this end, we can try implementing virtio-vhost:

Assume we want to bridge VMX and VMY using bridge in VMB.

- expose all of VMX and VMY memory as device BARs, or as some other
  region within VMB memory
- add interface to send vhost-user messages to VMB
  (and ack them) the messages include tables that
  translate from VMX/VMY physical to VMB physical.

The simplest guest driver then just copies from VMX TX ring to VMY RX
ring, and vice versa.
This will let you test performance somewhat easily.
When used as a linux netdev, we probably will have to do extra data copies,
at least initially.


The point is that you get full interoperability with existing
virtio, and test performance without rewriting everything
first.

One nice property is that KVM can log accesses for us.
By detecting VMB accesses to memory of VMX and forwarding
them to QEMU running VMX, we can make migration work
out of box.

This might also mean vringh code is reusable to make a linux
driver for this device - IIRC dirty logging was the biggest
hurdle to make vringh work well for vhost.


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