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 17:17:11 +0300

On Wed, May 02, 2018 at 10:00:14AM -0400, Vlad Yasevich wrote:
> On 05/02/2018 09:46 AM, Michael S. Tsirkin wrote:
> > 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".
> 
> Yes,  just as TUN_F_CSUM means that tun device can handle packets with
> partial tcp/udp checksum.
> 
> > 
> > Now what is the implication for macvtap? 
> 
> The implication is exactly the same as for TUN_F_CSUM.  If the
> flag is set on the macvtap device, the TX checksum feature is
> turned on.

I guess I will have to go back and re-read that code - I do not remember
what does TUN_F_CSUM does by now.  Here is a quick question that might
help me:

Let's assume userspace does not set TUN_F_SCTP_CSUM and device
does not calculate the checksums either. I would expect that with
macvtap this then behaves exactly like now with no overhead.

> 
> > And why  are
> > you setting NETIF_F_SCTP_CRC which is a flag
> > that affects packets sent by guest to host?
> 
> Mainly its because we are using just 1 flag to control checksum
> offloading and we need to be able control both tx and rx paths.

Well that's not really the case I think. What we have is controls for tx
offloads for tun. That's TUN_F_CSUM.
there are no rx offloads - userspace can send what it wants.

These are supposed to translate to rx offloads for macvtap.
tx offloads shouldn't be affected at all.

Maybe that's not the case - as I said need to go back and check.
Will try to find the time in the next couple of days.

> What you are suggesting that we either invert what TUN_F_CSUM
> is doing in macvtap case, or have another flag that lets us control
> TX and RX paths separately.
> 
> Either case, that would be separate work.
> -vlad

So assuming TUN_F_CSUM affects tx for macvtap do you
agree it's a bug? And should we add to it with
TUN_F_SCTP_CSUM doing the same?

> > 
> > 
> >>>
> >>>
> >>>>  	/* 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