KubeVirt with libvirt 9.0.0: Requested operation is not valid: The tap0 interface already exists

[Martin Kletzander]

On Thu, Jan 19, 2023 at 01:36:55PM +0100, Vasily Ulyanov wrote:
>Looking at the code now.
>
>src/util/virnetdevtap.c (qemuInterfaceEthernetConnect):
>
>    if (net->managed_tap == VIR_TRISTATE_BOOL_NO) {
>        ...
>    } else {
>        ...
>        tap_create_flags |= VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING;
>        ...
>    }
>
>Shouldn't it be the other way around? I.e. set
>VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING flag in case managed='no'?
>
>That is also mentioned in the commit message:
>
>  The only exception is <interface type='ethernet'/> with <target
>  managed='no'/> where we specifically want to let users use
>  pre-created TAP device and basically not touch it at all.
>

That makes sense and is probably the issue, I'll leave that to the
original author to decide (Cc'd), but in that case we also need to
change the following in the aforementioned patch [1]:

VIR_NETDEV_TAP_CREATE_ALLOW_EXISTING
  *     - The device creation fails if @ifname already exists

>
>On 19/01/2023 11:25, Vasily Ulyanov wrote:
>> Hi All,
>>
>> I am now trying to run our downstream KubeVirt build with a recent libvirt 9.0.0
>> and I see a problem that libvirt refuses to start a VM if it is configured to
>> use a pre-created tap device:
>>
>>   virError(Code=55, Domain=0, Message='Requested operation is not valid: The
>> tap0 interface already exists')
>>
>> After looking at the git history I found the bellow commit [1] which explicitly
>> forbids such a usecase. This basically breaks KubeVirt since libvirt there runs
>> unprivileged and can only use the existing tap's.
>>
>> I haven't yet look deeper into the original issue linked to the commit. Just
>> wanted to raise this here first. Perhaps someone can provide feedback on that.
>> What about some alternative approach instead? Or maybe it was discussed already...
>>
>> BTW, I tested with the reverted commit and it was successful.
>>
>> [1]
>> https://gitlab.com/libvirt/libvirt/-/commit/a2ae3d299cf9c5ada8aa42ec4271748eb479dc27
>>
>
>-- 
>Vasily Ulyanov <vulyanov at suse.de>
>Software Engineer, SUSE Labs Core
>
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