netdev - Re: [PATCH v4 bpf-next 3/3] veth: allow jumbo frames in xdp mode

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Message-ID: <YioT2TGc8M42V2K2@lore-desk>
Date:   Thu, 10 Mar 2022 16:06:01 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, brouer@...hat.com,
        pabeni@...hat.com, echaudro@...hat.com, toshiaki.makita1@...il.com,
        andrii@...nel.org
Subject: Re: [PATCH v4 bpf-next 3/3] veth: allow jumbo frames in xdp mode

> Lorenzo Bianconi <lorenzo@...nel.org> writes:
> 
> > Allow increasing the MTU over page boundaries on veth devices
> > if the attached xdp program declares to support xdp fragments.
> > Enable NETIF_F_ALL_TSO when the device is running in xdp mode.
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> > ---
> >  drivers/net/veth.c | 28 +++++++++++++++++-----------
> >  1 file changed, 17 insertions(+), 11 deletions(-)
> >
> > diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> > index 47b21b1d2fd9..c5a2dc2b2e4b 100644
> > --- a/drivers/net/veth.c
> > +++ b/drivers/net/veth.c
> > @@ -293,8 +293,7 @@ static int veth_forward_skb(struct net_device *dev, struct sk_buff *skb,
> >  /* return true if the specified skb has chances of GRO aggregation
> >   * Don't strive for accuracy, but try to avoid GRO overhead in the most
> >   * common scenarios.
> > - * When XDP is enabled, all traffic is considered eligible, as the xmit
> > - * device has TSO off.
> > + * When XDP is enabled, all traffic is considered eligible.
> >   * When TSO is enabled on the xmit device, we are likely interested only
> >   * in UDP aggregation, explicitly check for that if the skb is suspected
> >   * - the sock_wfree destructor is used by UDP, ICMP and XDP sockets -
> > @@ -302,11 +301,13 @@ static int veth_forward_skb(struct net_device *dev, struct sk_buff *skb,
> >   */
> >  static bool veth_skb_is_eligible_for_gro(const struct net_device *dev,
> >  					 const struct net_device *rcv,
> > +					 const struct veth_rq *rq,
> >  					 const struct sk_buff *skb)
> >  {
> > -	return !(dev->features & NETIF_F_ALL_TSO) ||
> > -		(skb->destructor == sock_wfree &&
> > -		 rcv->features & (NETIF_F_GRO_FRAGLIST | NETIF_F_GRO_UDP_FWD));
> > +	return rcu_access_pointer(rq->xdp_prog) ||
> > +	       !(dev->features & NETIF_F_ALL_TSO) ||
> > +	       (skb->destructor == sock_wfree &&
> > +		rcv->features & (NETIF_F_GRO_FRAGLIST | NETIF_F_GRO_UDP_FWD));
> >  }
> >  
> >  static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
> > @@ -335,7 +336,7 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
> >  		 * Don't bother with napi/GRO if the skb can't be aggregated
> >  		 */
> >  		use_napi = rcu_access_pointer(rq->napi) &&
> > -			   veth_skb_is_eligible_for_gro(dev, rcv, skb);
> > +			   veth_skb_is_eligible_for_gro(dev, rcv, rq, skb);
> >  	}
> >  
> >  	skb_tx_timestamp(skb);
> > @@ -1525,9 +1526,14 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> >  			goto err;
> >  		}
> >  
> > -		max_mtu = PAGE_SIZE - VETH_XDP_HEADROOM -
> > -			  peer->hard_header_len -
> > -			  SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> > +		max_mtu = SKB_WITH_OVERHEAD(PAGE_SIZE - VETH_XDP_HEADROOM) -
> > +			  peer->hard_header_len;
> 
> Why are we no longer accounting the size of the skb_shared_info if the
> program doesn't support frags?

doing so we do not allow packets over page boundaries (so non-linear xdp_buff)
if the attached program does not delclare to support them, right?

> 
> > +		/* Allow increasing the max_mtu if the program supports
> > +		 * XDP fragments.
> > +		 */
> > +		if (prog->aux->xdp_has_frags)
> > +			max_mtu += PAGE_SIZE * MAX_SKB_FRAGS;
> > +
> >  		if (peer->mtu > max_mtu) {
> >  			NL_SET_ERR_MSG_MOD(extack, "Peer MTU is too large to set XDP");
> >  			err = -ERANGE;
> > @@ -1549,7 +1555,7 @@ static int veth_xdp_set(struct net_device *dev, struct bpf_prog *prog,
> >  		}
> >  
> >  		if (!old_prog) {
> > -			peer->hw_features &= ~NETIF_F_GSO_SOFTWARE;
> > +			peer->hw_features &= ~NETIF_F_GSO_FRAGLIST;
> 
> The patch description says we're enabling TSO, but this change enables a
> couple of other flags as well. Also, it's not quite obvious to me why
> your change makes this possible? Is it because we can now execute XDP on
> a full TSO packet at once? Because then this should be coupled to the
> xdp_has_frags flag of the XDP program? Or will the TSO packet be
> segmented before it hits the XDP program? But then this change has
> nothing to do with the rest of your series?

actually tso support is not mandatory for this feature (even if it is probably
meaningful). I will drop it from v5 and we can take care of it in a susequent
patch.

Regards,
Lorenzo

> 
> Please also add this explanation to the commit message :)
> 
> -Toke
> 

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