Re: [dpdk-dev] [PATCH v8 10/10] app/testpmd:test VxLAN Tx checksum offload

["Liu, Jijiang" <jijiang.liu@intel.com>]

Hi,

> -----Original Message-----
> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
> Sent: Monday, November 17, 2014 7:22 PM
> To: Liu, Jijiang; Thomas Monjalon
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v8 10/10] app/testpmd:test VxLAN Tx checksum
> offload
> 
> Hi Jijiang,
> 
> On 11/17/2014 07:52 AM, Liu, Jijiang wrote:
> > Anyway, I explain the checksum mechanism here again.
> >
> > In my VXLAN patch set, for an VXLAN packet TX checksum offload,  the logics
> below:
> >
> > 1. only set outer L3/L4 header TX checksum
> >     tx_checksum set 0x3(0r 0x1) 0
> >   In this case, the PKT_TX_VXLAN_CKSUM flag is not set as we don't set
> > inner flags(PKT_TX_IPV4_CSUM, PKT_TX_UDP_CKSUM), so we don't need to
> change inner ones, driver think it is the ordinary packet, HW will do outer L3/L4
> checksum offload.
> 
> Let's take an example with the following packet:
> Ether / IP / UDP / VxLAN / Ether / IP / UDP / data
> 
> The original behavior (without your vxlan patches), which still works today, is to
> select inner or outer using the m->l2_len field:
> 
>   - checksum outer IP + UDP
>     m->l2_len=14 m->l3_len=20
>     flags=PKT_TX_IP_CKSUM PKT_TX_UDP_CKSUM
> 
>   - checksum inner IP + UDP
>     m->l2_len=64 m->l3_len=20
>     flags=PKT_TX_IP_CKSUM PKT_TX_UDP_CKSUM
>     of course, the packet is valid only if the outer IP checksum is
>     already correct and outer UDP checksum is 0
> 
> If i40e does not act like this, it does not follow the previous API.

No,  i40e follows this.

> > 2. only set inner L3/L4 header TX checksum
> >     tx_checksum set 0x30 0
> >   In this case, the PKT_TX_VXLAN_CKSUM flag is set, so driver think it is VXLAN
> packet, and we don't need to change outer ones because we don't set outer flags
> here (PKT_TX_IPV4_CSUM, PKT_TX_UDP_CKSUM).


> As explained above, there is no need to set the PKT_TX_VXLAN_CKSUM if you
> only want to set the inner L3/L4 checksum.
> This was already working like this
> before your patches, as long as l2_len and l3_len are set properly in the mbuf
> (l2_len should include the outer headers).

Does VXLAN TX checksum offload or ordinary L2 packet TX checksum offload work?
Have you ever tested it on a NIC that supports VXLAN. 
  
The PKT_TX_VXLAN_CKSUM flag meaning just tell driver this is encapsulation packet, so driver should set TX checksum offload for the packet using outer l2/l3 len, inner l2/l3 len and tunneling header length.

If you don't like this flag name, I can change it for  PKT_TX_TUNNEL_CKSUM, which have more generic meaning.

> Moreover, PKT_TX_IPV4_CSUM, PKT_TX_UDP_CKSUM, ... are not "outer flags".
> They are hardware checksum flags, and before your vxland patch, they concerned
> the headers referenced by m->l2_len and m->l3_len.

Actually, the  key point of debate is that you still think the l2_len filed and the l3_len filed  in mbuf are inner part in the case of tunneling, right?  If yes, let me explain what I thought.

As you know, NIC itself is not responsible for packet decapsulation / encapsulation at all. It sends and receives the whole packet, not only for inner part in the case of tunneling. The translation from receive descriptor to mbuf structure is also for the whole packet. And these fields defined in mbuf structure are also for the whole packet, no matter it is tunneling or non-tunneling.

1) We assume that a NIC can't  recognize VXLAN packet, when a packet  with the format  outer IP / outer UDP / VxLAN / Ether / inner IP / inner UDP / data is received, 
 do you think whether l2  header and l3 header length of this packet is outer or inner,  according to my understanding, I think it is outer, and m->l2_len and m->l3_len is also outer. Do you agree?

2) We also assume that a NIC can  recognize VXLAN packet,  but there is no difference between 1)  and 2) on data in mbuf before patching my VXLAN patch, so I also think  m->l2_len and m->l3_len is outer.  Do you agree?
After patching my VXLAN, the inner_l2_len and inner_l3_len were used to stand for inner header part.


> With your vxlan patch, it changed without beeing documented. These flags use
> either (m->l2_len, m->l3_len) or (m->inner_l2_len,
> m->inner_l3_len), which is not a good idea in my opinion.

The PKT_RX_IPV4_HDR  definition is listed below,
#define PKT_RX_IPV4_HDR      (1ULL << 5)  /**< RX packet with IPv4 header. */
I don't think it just stand for inner IP TX checksum offload, I just extend its usage scope in the case of tunneling.  

> > 3. set outer L3/L4 TX checksum and inner L3&L4 TX checksum tx_checksum
> > set 0x31(0x33) 0 in this case, the PKT_TX_VXLAN_CKSUM flag is set,
> > driver think it is VXLAN packet, and we need to change outer and inner as both
> outer and inner flags are set.
> 
> Here you are talking about test pmd flags. You do not describe the mbuf API:
> PKT_TX_* flags and lengths values that need to be set (l2_len, l3_len, ...) and to
> what they refer to.
> 
> I think if you want to explain the vxlan checksum offload mbuf API, you should not
> talk about the testpmd flags as:
> - they don't match the mbuf flags
> - they have undocumented (or uncoherent) behavior in the csumonly
>   forward engine
> 
> After several exchanges about this vxlan API.
> Unfortunately, it is still vague and obscure to me.

As to tunneling packet TX checksum offload, please don't think it is only an inner or outer part.
You should regard it as whole part.

> So here is a proposition of API documentation that looks understandable. Maybe
> it is easier to change the code to match this API:
> 
Ok, thanks.

> 
> PKT_TX_IP_CKSUM flag enables hardware computation of IP cksum. To use it:
> - fill l2_len and l3_len in mbuf
> - set the flag PKT_TX_IP_CKSUM
> - set the ip checksum to 0 in IP header
> See (1) and (2).
> 
> One value among PKT_TX_L4_NO_CKSUM, PKT_TX_UDP_CKSUM,
> PKT_TX_TCP_CKSUM and PKT_TX_SCTP_CKSUM can be assigned to the bits of
> PKT_TX_L4_MASK. These flags are used to offload the L4 checksum in hardware.
> The user requires to:
> - fill l2_len and l3_len in mbuf
> - set the flags PKT_TX_TCP_CKSUM, PKT_TX_SCTP_CKSUM or
>   PKT_TX_UDP_CKSUM
> - calculate the pseudo header checksum and set it in the L4
>   header (only for TCP or UDP). See rte_ipv4_phdr_cksum() and
>   rte_ipv6_phdr_cksum().  For SCTP, set the crc field to 0.
> See (1) and (2).
> 
> The PKT_TX_TCP_SEG flag can be set to enable TCP segmentation offload for a
> packet to be transmitted on hardware supporting
> TSO:
> - set the PKT_TX_TCP_SEG flag in mbuf->ol_flags (this flag
>   implies PKT_TX_TCP_CKSUM)
> - if it's IPv4, set the PKT_TX_IP_CKSUM flag and write the IP
>   checksum to 0 in the packet
> - fill the mbuf offload information: l2_len, l3_len, l4_len,
>   tso_segsz
> - calculate the pseudo header checksum without taking ip_len in
>   accound, and set it in the TCP header. Refer to
>   rte_ipv4_phdr_cksum() and rte_ipv6_phdr_cksum() that can be
>   used as helpers.
> See (1) and (2).
> 
> (1) In case the packet is an encapsulated packet, the m->l2_len
>     field can include all the outer tunnel headers. These headers
>     will remain unmodified by the hardware.
> 
> (2) If outer_l2_len and outer_l3_len are not 0, the beginning of
>     the inner headers is relative to outer_l2_len + outer_l3_len.
> 
> 
> [To replace the PKT_TX_VXLAN_CKSUM, we introduce 2 new flags]
> 
> PKT_TX_OUTER_IP_CKSUM flag enables hardware computation of IP cksum in
> outer headers. To use it:
> - fill outer_l2_len and outer_l3_len in mbuf
> - set the flag PKT_TX_OUTER_IP_CKSUM
> - set the ip checksum to 0 in outer IP header
> 
> One value among PKT_TX_OUTER_L4_NO_CKSUM,
> PKT_TX_OUTER_UDP_CKSUM, PKT_TX_OUTER_TCP_CKSUM and
> PKT_TX_OUTER_SCTP_CKSUM can be assigned to the bits of PKT_TX_L4_MASK.
> These flags are used to offload the outer L4 checksum in hardware.
> The user requires to:
> - fill outer_l2_len and outer_l3_len in mbuf
> - set the flags PKT_TX_OUTER_TCP_CKSUM, PKT_TX_OUTER_SCTP_CKSUM or
>   PKT_TX_OUTER_UDP_CKSUM
> - calculate the pseudo header checksum and set it in the outer L4
>   header (only for TCP or UDP). See rte_ipv4_phdr_cksum() and
>   rte_ipv6_phdr_cksum().  For SCTP, set the crc field to 0.

Good. You provide a common approach.

Actually, I have another common approach,
1. Change PKT_TX_VXLAN_CKSUM to PKT_TX_TUNNEL_CKSUM
2. Add field "uint8_t tun_header_len "(tunneling header length, for example, GRE header )into mbuf structure.
After above change, the API can supports other tunnels.

> 
> This proposition has several advantages:
> - it is documented :)
> - the API is straightforward: inner and outer work in the same
>   manner.
> - the API already supports other tunnels (IPIP, GRE, STT, ...)
> - adding m->outer_* fields allows to keep the same semantic for
>   the existing flags. Indeed, it does not map linux skb, but this
>   is not an argument. Moreover, linux does not seem to support
>   hardware tx checksum of outer+inner headers.

Just as I have mentioned in the previous email,  Linux have already supported hardware tx checksum of outer+inner headers for i40e.

> Regards,
> Olivier