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[PATCH/RFC 0/1] Vhost User Cross Cable: Intro

From: V.
Subject: [PATCH/RFC 0/1] Vhost User Cross Cable: Intro
Date: Wed, 8 Jan 2020 02:54:30 +0100
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.1

Hi List,

For my VM setup I tend to use a lot of VM to VM single network links to do 
routing, switching and bridging in VM's instead of the host.
Also stemming from a silly fetish to sometimes use some OpenBSD VMs as 
firewall, but that is besides the point here.
I am using the standard, tested and true method of using a whole bunch  of 
bridges, having 2 vhost taps each.
This works and it's fast, but it is a nightmare to manage with all the 
interfaces on the host.

So, I looked a bit into how I can improve this, basically coming down to "How 
to connect 2 VM's together in a really fast and easy way".
This however, is not as straightforward as I thought, without going the whole 
route of OVS/Snabb/any other big feature bloated
software switch.
Cause really, all I want is to connect 2 VM's in a fast and easy way. Shouldn't 
be that hard right?

Anyways, I end up finding tests/vhost-user-bridge.c, which is very nicely doing 
half of what I wanted.
After some doubling of the vhosts and eliminating udp, I came up with a Vhost 
User Cross Cable. (patch in next post).
It just opens 2 vhost sockets instead of 1 and does the forwarding between them.
A terrible hack and slash of vhost-user-bridge.c, probably now with bugs 
causing the dead of many puppies and the end of humanity,
but it works!

However... I now am left with some questions, which I hope some of you can 

I looked, googled, read and tried things, but it is likely that I am an 
complete and utter moron and my google-fu has just been awful...
Very likely... But is there really no other way then I have found to just link 
up 2 QEMU's in a fast non-bridge way? (No, not sockets.)
Not that OVS and the likes are not fine software, but do we really need the 
whole DPDK to do this?

In the unlikely chance that I'm not an idiot, then I guess now we have a nice 
simple cross cable.
However, I am still a complete vhost/virtio idiot who has no clue how it works 
and just randomly brute-forced code into submission.
Maybe not entirely true, but I would still appreciate it very much if someone 
with more knowledge into vhost to have a quick look at
how things are done in cc.

Specifically this monstrosity in TX (speed_killer is a 1MB buffer and kills any 
  ret = iov_from_buf(sg, num, 0, speed_killer,
                     iov_to_buf(out_sg, out_num, 0, speed_killer,
                                MIN(iov_size(out_sg, out_num), sizeof 

  vs. the commented:
  //ret = iov_copy(sg, num, out_sg, out_num, 0,
  //               MIN(iov_size(sg, num), iov_size(out_sg, out_num)));

The first is obviously a quick fix to get things working, however, in my meager 
understanding, should the 2nd one not work?
Maybe I'm messing up my vectors here, or I am messing up my understanding of 
iov_copy, but shouldn't the 2nd form be the way to zero

Now if Cross Cable is actually a new and (after a code-rewrite of 10) a viable 
way to connect 2 QEMU's together, could I actually
suggest a better way?
I am thinking of a '-netdev vhost-user-slave' option to connect (as client or 
server) to a master QEMU running '-netdev vhost-user'.
This way there is no need for any external program at all, the master can have 
it's memory unshared and everything would just work
and be fast.
Also the whole thing can fall back to normal virtio if memory is not shared and 
it would even work in pure usermode without any
context switch.

Building a patch for this idea I could maybe get around to, don't clearly have 
an idea how much work this would be but I've done
crazier things.
But is this is something that someone might be able to whip up in an hour or 
two? Someone who actually does have a clue about vhost
and virtio maybe? ;-)

Hacking together cc from bridge I noticed the use of container_of() to get from 
vudev to state in the vu callbacks.
Would it be an idea to add a context pointer to the callbacks (possibly gotten 
from VuDevIface)?
And I know. First post and I have the forwardness to even suggest an API 
change! I know!
But it makes things a bit simpler to avoid globals and it makes sense to have 
some context in a callback to know what's going on,
right? ;-)

Last one, promise.
I'm very much in the church of "less software == less bugs == less security 
Running cc or a vhost-user-slave means QEMU has fast networking in usermode 
without the need for anything else then AF_UNIX + shared
So might it be possible to weed out any modern fancy stuff like the Internet 
Protocol, TCP, taps, bridges, ethernet and tokenring
from a kernel and run QEMU on that?
The idea here is a kernel with storage, a serial console, AF_UNIX and vfio-pci, 
only running QEMU.
Would this be feasible? Or does QEMU need a kernel which at least has a grasp 
of understanding of what AF_INET and ethernet is?
(Does a modern kernel even still support tokenring? No idea, Probably does.)

Finally, an example and some numbers.

Compiling and starting the cross cable:
make tests/vhost-user-cc
tests/vhost-user-cc -l /tmp/left.sock -r /tmp/right.sock

(Note, the cross cable will quit if one of the vm's quits, but the VM's will 
reconnect when cc starts again.)

2 VM's, host1 and host2, Linux guests, run like this:

/qemu/bin/qemu-system-x86_64 \
  -accel kvm -nodefaults -k en-us -vnc none -machine q35 -cpu host -smp 
8,cores=8 -m 2G -vga std \
  -object memory-backend-file,id=memory,mem-path=/hugetlbfs,share=on,size=2G \
  -numa node,memdev=memory \
  -drive if=none,cache=none,format=raw,aio=native,file=/dev/lvm/host1,id=sda \
  -device virtio-scsi-pci,id=scsi0 -device scsi-hd,drive=sda,bus=scsi0.0 \
  -chardev socket,id=left,path=/tmp/left.sock,reconnect=1 \

/qemu/bin/qemu-system-x86_64 \
  -accel kvm -nodefaults -k en-us -vnc none -machine q35 -cpu host -smp 
8,cores=8 -m 2G -vga std \
  -object memory-backend-file,id=memory,mem-path=/hugetlbfs,share=on,size=2G \
  -numa node,memdev=memory \
  -drive if=none,cache=none,format=raw,aio=native,file=/dev/lvm/host2,id=sda \
  -device virtio-scsi-pci,id=scsi0 -device scsi-hd,drive=sda,bus=scsi0.0 \
  -chardev socket,id=right,path=/tmp/right.sock,reconnect=1 \

First, speed via eth0 (bridged tap with vhost, host2 runs './iperf3 -s'):
  root@host1:~/iperf-3.1.3/src# ./iperf3 -c -i 1 -t 10
  [  4]   0.00-10.00  sec  10.7 GBytes  9.22 Gbits/sec                  receiver

Second, speed via eth1 (Vhost Cross Cable):
  root@host1:~/iperf-3.1.3/src# ./iperf3 -c -i 1 -t 10
  [  4]   0.00-10.00  sec  2.05 GBytes  1.76 Gbits/sec                  receiver

So, a factor of 6 slowdown against bridge. Not too bad, considering the bad 
iovec mem-copying I do.
Lots of room for improvement though, but at least for me it's also 5 times 
faster as socket.


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