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

[Vlad Yasevich <vyasevic@redhat.com>]

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 <vyasevic@redhat.com>
>>>> ---
>>>>  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.

> 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.

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

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