Re: [dpdk-dev] [PATCH] net/af_packet: try to reinsert the stripped vlan tag

[Tudor Cornea <tudor.cornea@gmail.com>]

Indeed, the vlan insertion could be a costly operation. We should probably
do it only if the user specifically asks to have the vlan tag in the packet.
Otherwise, af_packet PMD users might pay a price in terms of performance
for something they didn't ask for.

I was thinking of avoiding having to change the application in order to
re-insert the vlan tag.
Doing this operation inside the PMD driver seemed like a good fit.

Looking at the netvsc driver (drivers/net/netvsc), the vlan insertion is
guarded by a check to hv->vlan_strip

  if (!hv->vlan_strip && rte_vlan_insert(&m)) {

hv->vlan_strip seems to be initialized in hn_dev_configure() in the
following way

 hv->vlan_strip = !!(rxmode->offloads & DEV_RX_OFFLOAD_VLAN_STRIP);

while 'hv' seems to be stored in rte_eth_dev->data->dev_private

I am thinking of doing something similar for the af_packet PMD.
The 'pmd_internals' structure could potentially hold a field, say
vlan_strip', which could be initialized if the application enables the
DEV_RX_OFFLOAD_VLAN_STRIP in rxmode->offloads

This way, I'm thinking that the application could potentially control the
effect of vlan stripping for the af_packet PMD, in an uniform way, similar
to other PMDs.
Would this be considered an acceptable solution ?




On Tue, 31 Aug 2021 at 18:31, Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 8/20/2021 1:46 PM, Tudor Cornea wrote:
> > The af_packet pmd driver binds to a raw socket and allows
> > sending and receiving of packets through the kernel.
> >
> > Since commit bcc6d47903 [1], the kernel strips the vlan tags early in
> > __netif_receive_skb_core(), so we receive untagged packets while
> > running with the af_packet pmd.
> >
> > Luckily for us, the skb vlan-related fields are still populated from the
> > stripped vlan tags, so we end up having all the information
> > that we need in the mbuf.
> >
> > We would like to have the the vlan tag inside the mbuf.
> > Let's take a shot at it by trying to reinsert the stripped vlan tag.
> >
>
> PMD already sets 'mbuf->vlan_tci' and 'PKT_RX_VLAN | PKT_RX_VLAN_STRIPPED'
> flags, so application can be aware of the vlan tag and can consume it.
>
> Inserting the vlan tag back to packet is costly, what is the motivation to
> do so?
>
> > As a side note, something similar was done for the netvsc pmd.
> >
> > [1]
> https://github.com/torvalds/linux/commit/bcc6d47903612c3861201cc3a866fb604f26b8b2
> >
> > Signed-off-by: Tudor Cornea <tudor.cornea@gmail.com>
> > ---
> >  drivers/net/af_packet/rte_eth_af_packet.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/net/af_packet/rte_eth_af_packet.c
> b/drivers/net/af_packet/rte_eth_af_packet.c
> > index b73b211..d116583 100644
> > --- a/drivers/net/af_packet/rte_eth_af_packet.c
> > +++ b/drivers/net/af_packet/rte_eth_af_packet.c
> > @@ -148,6 +148,10 @@ eth_af_packet_rx(void *queue, struct rte_mbuf
> **bufs, uint16_t nb_pkts)
> >               if (ppd->tp_status & TP_STATUS_VLAN_VALID) {
> >                       mbuf->vlan_tci = ppd->tp_vlan_tci;
> >                       mbuf->ol_flags |= (PKT_RX_VLAN |
> PKT_RX_VLAN_STRIPPED);
> > +
> > +                     /* the kernel always strips the vlan tag, try to
> reinsert it */
> > +                     if (rte_vlan_insert(&mbuf))
> > +                             PMD_LOG(ERR, "Failed to reinsert vlan
> tag");
> >               }
> >
> >               /* release incoming frame and advance ring buffer */
> >
>
>