Re: [dpdk-dev] [PATCH v2 6/7] net/e1000: set min and max MTU for igb devices

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

> >
> > Hi Qi,
> >
> > > >
> > > > On 3/22/2019 1:01 PM, Ian Stokes wrote:
> > > > > This commit sets the min and max supported MTU values for igb
> > > > > devices via the eth_igb_info_get() function. Min MTU supported is
> > > > > set to ETHER_MIN_MTU and max mtu is calculated as the max packet
> > > > > length supported minus the transport overhead. To aid in these
> > > > > calculations a new MACRO 'E1000_ETH_OVERHEAD' has been introduced
> > > > > to consolidate overhead calculation and avoid duplication.
> > > > >
> > > > > Signed-off-by: Ian Stokes <ian.stokes@intel.com>
> > > > > ---
> > > > >  drivers/net/e1000/e1000_ethdev.h | 6 ++++++
> > > > >  drivers/net/e1000/igb_ethdev.c   | 7 +++++--
> > > > >  2 files changed, 11 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/net/e1000/e1000_ethdev.h
> > > > > b/drivers/net/e1000/e1000_ethdev.h
> > > > > index 94edff08e..3e74cd8fe 100644
> > > > > --- a/drivers/net/e1000/e1000_ethdev.h
> > > > > +++ b/drivers/net/e1000/e1000_ethdev.h
> > > > > @@ -89,6 +89,12 @@
> > > > >  	ETH_RSS_IPV6_UDP_EX)
> > > > >
> > > > >  /*
> > > > > + * The overhead from MTU to max frame size.
> > > > > + * Considering VLAN so a tag needs to be counted.
> > > > > + */
> > > > > +#define E1000_ETH_OVERHEAD (ETHER_HDR_LEN + ETHER_CRC_LEN +
> > > > > +VLAN_TAG_SIZE)
> > > >
> > > > As an overhead, following drivers set:
> > > > i40e: HDR + CRC + 2 * VLAN
> > > > ixgbe: HDR + CRC
> > > > e1000: HDR + CRC + VLAN
> > > >
> > > > I wonder if this difference is HW limitation, or driver limitation
> > > > or just implementation inconsistency.
> > >
> > > I think this is implementation inconsistency
> > >
> > > The NIC only accept Max Frame Size.
> > >
> > > The problem here is seems all of three setup are not perfect.
> > >
> > > HDR + CRC + 2 * VLAN - it may allow non vlan or single vlan packet that exceed
> > mtu.
> >
> > Hmm, wonder how?
> 
> I'm talking about the case:
> 
> Assume mtu = 1500,  we will set max frame size to 1500 + 14 + 4 + 2*4 = 1526
> Let's assume a non vlan packet with 1522 size, so its l2 payload will be 1504 that exceed the mtu,  but it will still be accepted, does it break
> the configure?

Of course it would, but as I can read the mail, we discussing overhead
to subtract from max_rx_pkt_len to report max allowable mtu.
From that perspective bigger overhead is more conservative and makes sure
our tx packet will never be bigger than max_rx_pkt_len.
Konstantin

> 
> >
> > > HDR + CRC - it may reject vlan or double vlan packet that follow mtu.
> > > HDR + CRC + VLAN ,  it may reject double vlan packet that follow mtu
> > >
> > > I agree it's better to keep consistent on all drivers, but before
> > > this, we may need to decide which one we should take :)
> > >
> > > Regards
> > > Qi
> > >
> > > >
> > > > Better to confirm it that it is not implementation inconsistency.
> > > >
> > > > Wenzhuo, Konstantin, Beilei, Qi,
> > > >
> > > > Can you please comment?
> > > >
> > > > Thanks,
> > > > ferruh