Re: [PATCH v7] net/tap: Allow jumbo frames

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 8 Aug 2022 17:38:21 +0200
Francesco Mancino <francesco.mancino@tutus.se> wrote:

> Thank you for the feedback. 
> 
> I am sorry for the email spam, it was a learning process.
> 
> I am not sure what results we would get by querying the MTU
> before configuring it, since we can (and do) set the MTU using
> SIOCSIFMTU.
> 
> On 2022-08-08 17:03, Stephen Hemminger wrote:
> > On Mon, 8 Aug 2022 16:49:44 +0200
> > Francesco Mancino <francesco.mancino@tutus.se> wrote:
> >   
> >> eth_dev_validate_mtu, introduced in 990912e676e, validates configured
> >> MTU plus overhead against max_rx_pktlen.
> >> Since TAP is a virtual device, it should support as big MTU as possible.
> >>
> >> Signed-off-by: Francesco Mancino <francesco.mancino@tutus.se>  
> > 	dev_info->min_rx_bufsize = 0;
> > 
> > 
> > Thanks for your patience.
> > 
> > Since tap is built on top of an existing kernel network device a more
> > complete solution would be to query the kernel device to find out what
> > its MTU is. 
> > 
> > Acked-by: Stephen Hemminger <stephen@networkplumber.org>
> >   
> 

There is an attribute IFLA_MAX_MTU reported by devices over netlink.

$ ip -d li show dev enp2s0
2: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq state UP mode DEFAULT group default qlen 1000
    link/ether d8:5e:d3:06:5d:13 brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 9194 addrgenmode none numtxqueues 1 numrxqueues 1 gso_max_size 64000 gso_max_segs 64 gro_max_size 65536 parentbus pci parentdev 0000:02:00.0

Notice maxmtu of 9194 reported.

Tap device already has a netlink infrastructure