Re: [Xen-devel] Xen 4.x / Linux 3.x (dom0 and HVM domU) and NIC handling

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Fri, Dec 02, 2011 at 11:11:38AM +0100, Stefan Bader wrote:
> On 01.12.2011 19:09, Pasi Kärkkäinen wrote:
> > On Thu, Dec 01, 2011 at 04:09:38PM +0100, Stefan Bader wrote:
> >> Moving to public discussion...
> >>
> >> This was found with Xen hypervisor version supporting device unplugging 
> >> and the
> >> domU kernel having net-/blkfront and pci platform built-in (or as module).
> >>
> >> The block device is defined as hda and the NIC type=ioemu (so theoretically
> >> guests without pv support would work, too).
> >>
> >> Since both drivers are present, the kernel tries to unplug the emulated 
> >> devices
> >> and succeeds. The blkfront driver detects the xvda device available in 
> >> parallel
> >> and is working ok.
> >>
> >> However the network interface does not work. There are entries present 
> >> under
> >> sysfs for the xenbus but trying to bring it up fails with errors. And also 
> >> there
> >> seems to be no mac address set (all zeros in sysfs).
> >> When the type=ioemu is removed in the configuration, this works.
> >>
> >> I have not much more debugging information beyond that, yet. But it sounds 
> >> a bit
> >> like NICs should behave the same as block devices. So if there is an 
> >> emulated
> >> device defined there will be an alternate paravirt interface for it and 
> >> after
> >> unplugging the emulated ones we end up with the pv ones.
> >> Is that something that can be seen with newer Xen versions, too (I am 
> >> using 4.1.1)?
> >>
> > 
> > Hey,
> > 
> > Have you seen?: 
> > http://wiki.xen.org/xenwiki/XenLinuxPVonHVMdrivers
> > 
> > Especially the following note:
> > "NOTE! If you have "type=ioemu" specified for the "vif"-line, PVHVM drivers 
> > WILL NOT work! Don't specify "type" parameter for the vif. (with type=ioemu 
> > the pvhvm nic in the VM will have mac address full of zeroes - and thus 
> > won't work!)."
> > 
> > "type=ioemu" is not needed, at least with xm/xend toolstack both HVM and 
> > PVHVM guests work OK without it.
> > 
> > -- Pasi
> > 
> Thanks Pasi,
> 
> hmm, so it is documented actually and thus sort of expected. Still it is
> confusing. For one driver it does not make a difference to use the form of an
> emulated device in the config, for the other it does. The xl stack works, the 
> xm
> stack does not.
> And then, ok this is probably a quite naive approach, it seemed to make sense 
> to
> go through the pain of always having potentially both interfaces available
> (emulated and pv) so in theory the same guest config can accommodate a guest 
> os
> supporting one or the other (or easily switch from one to the other). 
> Otherwise
> I would expect an emulated device only when I have hd? in the config and a pv
> device when I write xvd?.
> 

This works for both HVM and PVHVM (also mentioned on the wiki page):
vif = [ 'mac=00:16:5e:02:07:45, bridge=xenbr0, model=e1000' ]

So there's no need for "type=ioemu" option with xm/xend.

You can switch between HVM and PVHVM with:
xen_platform_pci=0|1

-- Pasi


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