Re: [PATCH] net/tap: do not include l2 header in gso size when compared with mtu

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 8 Mar 2022 22:35:18 +0800
Harold Huang <baymaxhuang@gmail.com> wrote:

> On Mon, Feb 28, 2022 at 4:27 PM Harold Huang <baymaxhuang@gmail.com> wrote:
> >
> > The gso size is calculated with all of the headers and payload. As a
> > result, the l2 header should not be included when comparing gso size
> > with mtu.
> >
> > Fixes: 050316a88313 ("net/tap: support TSO (TCP Segment Offload)")
> > Cc: stable@dpdk.org
> > Signed-off-by: Harold Huang <baymaxhuang@gmail.com>
> > ---
> >  drivers/net/tap/rte_eth_tap.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/tap/rte_eth_tap.c b/drivers/net/tap/rte_eth_tap.c
> > index f1b48cae82..2b561d232c 100644
> > --- a/drivers/net/tap/rte_eth_tap.c
> > +++ b/drivers/net/tap/rte_eth_tap.c
> > @@ -731,7 +731,7 @@ pmd_tx_burst(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
> >                                         mbuf_in->l4_len;
> >                         tso_segsz = mbuf_in->tso_segsz + hdrs_len;
> >                         if (unlikely(tso_segsz == hdrs_len) ||
> > -                               tso_segsz > *txq->mtu) {
> > +                               tso_segsz > *txq->mtu + mbuf_in->l2_len) {
> >                                 txq->stats.errs++;
> >                                 break;
> >                         }
> > --
> > 2.27.0
> >  
> 
> Hi, Jiayu,
> 
> This is the only example in the driver to use GSO. I think it is
> important for us to calculate a correct GSO size. I see you are the
> GSO lib maintainer, could you please help review this patch?

See Ophir's comment and address it in new version

Please note that max size is calculated in the same function (drivers/net/tap/rte_eth_tap.c) as:
 max_size = *txq->mtu + (RTE_ETHER_HDR_LEN + RTE_ETHER_CRC_LEN + 4);

Since tso_setgsz should not exceed the max packet size - the desired update should be:
                      tso_segsz > max_size

rather than the suggested one:

                      tso_segsz > *txq->mtu + mbuf_in->l2_len)