Re: consuming pre-created tap - with multiqueue

On Tue, Sep 22, 2020 at 01:48:08PM +0200, Miguel Duarte de Mora Barroso wrote: > Hello, > > On KubeVirt, we are trying to pre-create a tap device, then instruct > libvirt to consume it (via the type=ethernet , managed='no' > attributes). > > It works as expected, **unless** when we create a multi-queue tap device. > > The difference when creating the tap device is that we set the > multi-queue flag; libvirt throws the following error when consuming > it: > > ``` > LibvirtError(Code=38, Domain=0, Message='Unable to create tap device > tap0: Invalid argument') > ``` > > After digging a bit on the libvirt code (we're using libvirt 6.0.0), I > see this on the logs (immediately before the error): > {"component":"virt-launcher","level":"info","msg":"Enabling > IFF_VNET_HDR","pos":"virNetDevProbeVnetHdr:190","subcomponent":"libvirt","thread":"33" > ,"timestamp":"2020-09-22T10:34:29.335000Z"} > > I do not understand how it can try to set the VNET_HDR flag, since I > have not set it when I created it, which, as per [0] should only > happen when requested. Here's the tap device I'm creating: (output of > `ip tuntap show`) > - tap0: tap persist0x100 user 107 group 107 IIUC the kernel code correctly, the VNET_HDR flag is not required to be set when you first create the tap device - it appears to be permitted any time you open a file descriptor for it. AFAIK, there is no problem with VNET_HDR, as it is a standard flag we've set on all tap devices on Linux for 10 years.