Re: [dpdk-dev] [RFC] ethdev: add min/max MTU to device info

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> > From: dev on behalf of Stephen Hemminger
> > On Wed, 6 Feb 2019 14:05:34 +0100
> > Morten Brørup <mb@smartsharesystems.com> wrote:
> >
> > > Good work, Stephen.
> > >
> > > It should also be documented how PMDs should interpret this MTU.
> > >
> > > Obviously, a VLAN tagged Ethernet frame grows from 1518 to 1522 bytes incl. header and CRC, and should be allowed with an Ethernet
> MTU of 1500 bytes. There's even a #define ETHER_MAX_VLAN_FRAME_LEN for this, but that's as far as it goes...
> > >
> > > But how about frames with even larger headers, e.g. 4 MPLS labels makes a frame 16 bytes longer, i.e. it grows from 1518 to 1534
> bytes... is such a frame acceptable with an MTU of 1500 bytes?
> >
> > No. According to standard practice in Linux and FreeBSD, only the first VLAN tag is free.
> > After that any other headers count against MTU.
> 
> Thank you for the insights. Just to clarify:
> 1 VLAN tag is allowed for free.
> But on order to support two VLAN tags, the MTU must be increased by the size of one VLAN tag, because the first VLAN tag is free?
> Or must the MTU be increased by the size of two VLAN tags, because only the special case of exactly one VLAN tag is free?

Can we introduce new function at ehtdev API to query PMD frame size based on MTU?
Something like: rte_ethdev_mtu_to_frame_size(uint32_t mtu);
Provide default behavior and allow PMD to overwrite it?
Konstantin 

> 
> I wish details like this were documented... for the benefit of both PMD developers and DPDK users. Which is why I CC'ed the
> Documentatation and Ethernet API maintainers.
> 
> It reminds me of the previous discussion about some Ethernet PMD's not padding to minimum Ethernet frame size... DPDK users expecting a
> certain behavior based on Linux/BSD behavior, but not documented, so the PMD developers were unaware (or possibly disagreed).
> 
> We all know that the devil is in the details! Which is why documenting such details is important.
> 
> Outside the DPDK core libraries, the Ethernet PMDs are probably the closest to the DPDK core you are going to get.
> The core libraries have a bunch of test cases. Perhaps there should be more PMD test cases.
> 
> >
> > >
> > > According to Wikipedia (https://en.wikipedia.org/wiki/Maximum_transmission_unit), MTU describes the maximum payload size, and is
> not tied to the maximum frame size. However, the Linux kernel (at least the older versions I have been working with) incorrectly ties the
> MTU directly to the maximum frame size, so the MTU has to increased to support larger headers (e.g. QinQ or an MPLS stack). Do none, all
> or some DPDK PMDs suffer from the same error?
> > >
> >
> > Maximum Transmission Unit and Maximum Receive Unit (MRU) are separate. On most hardware they are the same but
> > there is variation. Some hardware can receive larger sizes than MTU and some have max receive length register.
> 
> Yes. This is also not obvious, although the name Maximum Transmission Unit strongly indicates that it does not apply to ingress. It took me
> by surprise a few years ago while working with the Linux kernel.
> 
> 
> 
>