Re: [Xen-users] VM bridge doesn't pass traffic

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2016-01-18 at 13:04 +0100, David Winterstein wrote:
> > I should have asked for these last time, sorry, but could you also post
> > your guest cfg file and the output of "xenstore-ls -fp".
> 
> You can view the config file here: http://pastebin.com/M3ZFT47Q
> It's pretty basic with only little changes after automatic creation by
> Xen.

Indeed, nothing untoward here.
> 
> The output of "xenstore-ls -fp": http://pastebin.com/VdtEbyNB

From this I can see that the front and backend network devices are talking
to each other and have reached the "connected" state (state = 4 under
/local/domain/2/device/vif/0 and the corresponding b/e dir)

> 
> 
> > Are you creating a PV or HVM guest?
> 
> If I'm getting it right from this source
> [http://serverfault.com/a/511938] and the following output, it is a
> "PV-HVM" guest:

Actually it looks like just PV, since your cfg file lacks the necessary
"builder = 'hvm'" which would make it HVM or PV-HVM.

Since Linux ~2.6.32 it hasn't been necessary to run a special kernel for
Xen (the so-called "pvops" support means a single binary can run native and
on Xen) so you typically won't see "xen" in the uname any more, contrary to
what that serverfault answer says.

> 
> root@www:~# uname -a
> Linux www 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u2
> (2016-01-02) x86_64 GNU/Linux
> root@www:~# lsmod | grep xen
> xen_netfrontÂÂÂÂÂÂÂÂÂÂÂ26038ÂÂ0
> xen_blkfrontÂÂÂÂÂÂÂÂÂÂÂ25614ÂÂ1

BTW, this is useful debug too since it shows that you have the netfront
module in the guest.

> > Could you also post any relevant logs from under /var/log/xen, mostly
> > the
> > ones with the domain's name in the filename, but also anything to do
> > with
> > hotplug if you have any files like that.
> 
> The logs are pretty empty:
> 
> [...]

Thanks, indeed not much of interest here :-/

> 
> > Lastly while I'm fishing for potential clues please could you post the
> > output of of "xl -vvv create <cfg>", the -vvv makes it much more
> > verbose.
> 
> Output of the create command: http://pastebin.com/7CqdS2Zd
> Output of the vm boot process (from xl console):
> http://pastebin.com/ZduwSmNn

Ah, here is something interesting:

[ÂÂÂÂ1.507834] xen_netfront: Initialising Xen virtual ethernet driver
[ÂÂÂÂ1.513754] xen_netfront: can't alloc rx grant refs
[ÂÂÂÂ1.513762] net eth0: only created 32 queues

If you use fewer vcpus (like 2 for example) does it work? If so then a
workaround would be to limit the number of queues to fewer than the number
of vcpus (the default). IIRC there were some bugs in this capability, I'm
hoping that Wei (cc-d) can remember the necessary dance.

An alternative to limiting the queues might be to increase the maximum
number of pages each guest can use for grant tables (i.e. increase the
number of grants). This can be done with theÂgnttab_max_nr_frames
hypervisor command line option. (SeeÂhttp://xenbits.xen.org/docs/4.4/misc/x
en-command-line.html).

I think the default is 32, so you could e.g. try doubling it.

Ian.

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