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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

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?.

-Stefan

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