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Re: consuming pre-created tap - with multiqueue



On Wed, Sep 23, 2020 at 05:44:28PM +0100, Daniel P. Berrangé wrote:
> 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.

Looking at the kernel code, you need to set the MULTI_QUEUE flag
at time you create the device and also set it when opening the
device. In tun_set_iff():


                if (!!(ifr->ifr_flags & IFF_MULTI_QUEUE) !=
                    !!(tun->flags & IFF_MULTI_QUEUE))
                        return -EINVAL;


so if you've configured QEMU to use multiqueue, the you need
to use:

  $ ip tuntap add dev mytap mode tap vnet_hdr multi_queue

actually vnet_hdr doesn't matter as that can be set on the fly
but multi_queue is mandatory.  Without it, I get the same EINVAL
error as you mention.


Regards,
Daniel
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