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

[Francesco Mancino <francesco.mancino@tutus.se>]

Interesting, did not know about that attribute.

Tested some tap devices on my machine and get a maxmtu value close to
max uint16, way bigger than RTE_ETHER_MAX_JUMBO_FRAME_LEN.

I do not know if will have time to code and test using netlink to set this limit,
for the time being i think RTE_ETHER_MAX_JUMBO_FRAME_LEN is better than the previous limit.

On 2022-08-08 18:42, Stephen Hemminger wrote:
> 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
>