Subject: Re: [PATCH V2 net-next 5/6] macvlan/macvtap: Add support for SCTP checksum offload.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 2 May 2018 16:46:17 +0300

On Wed, May 02, 2018 at 09:27:00AM -0400, Vlad Yasevich wrote:
> On 05/01/2018 11:24 PM, Michael S. Tsirkin wrote:
> > On Tue, May 01, 2018 at 10:07:38PM -0400, Vladislav Yasevich wrote:
> >> Since we now have support for software CRC32c offload, turn it on
> >> for macvlan and macvtap devices so that guests can take advantage
> >> of offload SCTP checksums to the host or host hardware.
> >>
> >> Signed-off-by: Vladislav Yasevich <[email-masked]>
> >> ---
> >>  drivers/net/macvlan.c | 5 +++--
> >>  drivers/net/tap.c     | 8 +++++---
> >>  2 files changed, 8 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
> >> index 725f4b4..646b730 100644
> >> --- a/drivers/net/macvlan.c
> >> +++ b/drivers/net/macvlan.c
> >> @@ -834,7 +834,7 @@ static struct lock_class_key macvlan_netdev_addr_lock_key;
> >>  
> >>  #define ALWAYS_ON_OFFLOADS \
> >>  	(NETIF_F_SG | NETIF_F_HW_CSUM | NETIF_F_GSO_SOFTWARE | \
> >> -	 NETIF_F_GSO_ROBUST | NETIF_F_GSO_ENCAP_ALL)
> >> +	 NETIF_F_GSO_ROBUST | NETIF_F_GSO_ENCAP_ALL | NETIF_F_SCTP_CRC)
> >>  
> >>  #define ALWAYS_ON_FEATURES (ALWAYS_ON_OFFLOADS | NETIF_F_LLTX)
> >>  
> >> @@ -842,7 +842,8 @@ static struct lock_class_key macvlan_netdev_addr_lock_key;
> >>  	(NETIF_F_SG | NETIF_F_HW_CSUM | NETIF_F_HIGHDMA | NETIF_F_FRAGLIST | \
> >>  	 NETIF_F_GSO | NETIF_F_TSO | NETIF_F_LRO | \
> >>  	 NETIF_F_TSO_ECN | NETIF_F_TSO6 | NETIF_F_GRO | NETIF_F_RXCSUM | \
> >> -	 NETIF_F_HW_VLAN_CTAG_FILTER | NETIF_F_HW_VLAN_STAG_FILTER)
> >> +	 NETIF_F_HW_VLAN_CTAG_FILTER | NETIF_F_HW_VLAN_STAG_FILTER | \
> >> +	 NETIF_F_SCTP_CRC)
> >>  
> >>  #define MACVLAN_STATE_MASK \
> >>  	((1<<__LINK_STATE_NOCARRIER) | (1<<__LINK_STATE_DORMANT))
> >> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> >> index 9b6cb78..2c8512b 100644
> >> --- a/drivers/net/tap.c
> >> +++ b/drivers/net/tap.c
> >> @@ -369,8 +369,7 @@ rx_handler_result_t tap_handle_frame(struct sk_buff **pskb)
> >>  		 *	  check, we either support them all or none.
> >>  		 */
> >>  		if (skb->ip_summed == CHECKSUM_PARTIAL &&
> >> -		    !(features & NETIF_F_CSUM_MASK) &&
> >> -		    skb_checksum_help(skb))
> >> +		    skb_csum_hwoffload_help(skb, features))
> >>  			goto drop;
> >>  		if (ptr_ring_produce(&q->ring, skb))
> >>  			goto drop;
> >> @@ -945,6 +944,9 @@ static int set_offload(struct tap_queue *q, unsigned long arg)
> >>  		}
> >>  	}
> >>  
> >> +	if (arg & TUN_F_SCTP_CSUM)
> >> +		feature_mask |= NETIF_F_SCTP_CRC;
> >> +
> > 
> > so this still affects TX, shouldn't this affect RX instead?
> 
> There is no bit to set on the RX path just like there is no bit to set on the RX patch
> for TUN_F_CSUM.
> 
> We only invert TSO offloads, not checksum offloads as the comment below states.
> For checksum,  macvtap has to compute the checksum itself in tap_handle_frame() above.
> It uses tx feature bits to see if needs do to the checksum.
> 
> If you think we need another flag to macvtap to control RXCSUM, that would need to be
> separate and cover standard TCP checksum as well.
> 
> -vlad

Confused. What is the meaning of TUN_F_SCTP_CSUM? I assume this is
a way for userspace to tell tun device: "I can handle
packets without SCTP checksum, pls send them my way".

Now what is the implication for macvtap? And why  are
you setting NETIF_F_SCTP_CRC which is a flag
that affects packets sent by guest to host?

> > 
> > 
> >>  	/* tun/tap driver inverts the usage for TSO offloads, where
> >>  	 * setting the TSO bit means that the userspace wants to
> >>  	 * accept TSO frames and turning it off means that user space
> >> @@ -1077,7 +1079,7 @@ static long tap_ioctl(struct file *file, unsigned int cmd,
> >>  	case TUNSETOFFLOAD:
> >>  		/* let the user check for future flags */
> >>  		if (arg & ~(TUN_F_CSUM | TUN_F_TSO4 | TUN_F_TSO6 |
> >> -			    TUN_F_TSO_ECN | TUN_F_UFO))
> >> +			    TUN_F_TSO_ECN | TUN_F_UFO | TUN_F_SCTP_CSUM))
> >>  			return -EINVAL;
> >>  
> >>  		rtnl_lock();
> >> -- 
> >> 2.9.5