[dpdk-dev] about rx checksum flags

[Chandran, Sugesh]

Hi Olivier,

Thank you for working on this..
A comment on the proposal is given below,
 

Regards
_Sugesh

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ananyev,
> Konstantin
> Sent: Wednesday, June 1, 2016 10:07 AM
> To: Stephen Hemminger <stephen at networkplumber.org>; Olivier MATZ
> <olivier.matz at 6wind.com>
> Cc: Yuanhan Liu <yuanhan.liu at linux.intel.com>; dev at dpdk.org; Richardson,
> Bruce <bruce.richardson at intel.com>; Adrien Mazarguil
> <adrien.mazarguil at 6wind.com>; Tan, Jianfeng <jianfeng.tan at intel.com>
> Subject: Re: [dpdk-dev] about rx checksum flags
> 
> 
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> > Sent: Tuesday, May 31, 2016 11:03 PM
> > To: Olivier MATZ
> > Cc: Yuanhan Liu; dev at dpdk.org; Ananyev, Konstantin; Richardson, Bruce;
> > Adrien Mazarguil; Tan, Jianfeng
> > Subject: Re: [dpdk-dev] about rx checksum flags
> >
> > On Tue, 31 May 2016 22:58:57 +0200
> > Olivier MATZ <olivier.matz at 6wind.com> wrote:
> >
> > > Hi Stephen,
> > >
> > > On 05/31/2016 10:28 PM, Stephen Hemminger wrote:
> > > > On Tue, 31 May 2016 21:11:59 +0200 Olivier MATZ
> > > > <olivier.matz at 6wind.com> wrote:
> > > >
> > > >>
> > > >>
> > > >> On 05/31/2016 10:09 AM, Yuanhan Liu wrote:
> > > >>> On Mon, May 30, 2016 at 05:26:21PM +0200, Olivier Matz wrote:
> > > >>>>  PKT_RX_L4_CKSUM_NONE: the L4 checksum is not correct in the
> > > >>>> packet  data, but the integrity of the L4 header is verified.
> > > >>>>   -> the application can process the packet but must not verify the
> > > >>>>      checksum by sw. It has to take care to recalculate the cksum
> > > >>>>      if the packet is transmitted (either by sw or using tx
> > > >>>> offload)
> > > >>>
> > > >>> I like the explanation you made at [1] better :)
> > > >>>
> > > >>> So in general, I think this proposal is good to have.
> > > >>
> > > >> Thanks everyone for your feedback.
> > > >>
> > > >> I'll try to send a first patch proposition soon.
> > > >>
> > > >> Regards,
> > > >> Olivier
> > > >
> > > > I think it is time to ditch the old definitions of Rx checksum and
> > > > instead use something more compatiable with virtio (and Linux). I.e
> have three values
> > > >   1) checksum is know good for packet contents
> > > >   2) checksum value one's complement for packet contents
> > > >   3) checksum is undetermined
> > > > The original definition seems to be Intel HW centric and applies
> > > > to a limited range of devices making it unusable by general application.
> > > >
> > > > Break the ABI, and ditch the old values (ok mark
> > > > PKT_RX_L4_CKSUM_BAD as __deprecated and remove all usage).
> > > >
> > >
> > > Don't you think knowing that a checksum is bad could be useful?
> >
> > Not really. They should be mark as undetermined, then software can
> > recheck for the possibly buggy hardware.
> 
> Hmm, I don't see the point here.
> If the HW clearly reports that checksum is invalid (not unknown), why SW has
> to assume it is ' undetermined' and recheck it?
> To me that means just wasted cycles.
> In general, it sounds like really strange approach to me:
> write your SW with assumption that all HW you are going to use will not work
> correctly.
> 
> >
> > > In that case the application can drop/log the packet without any
> > > additional cpu cost.
> > >
> > > What do you mean by beeing unusable by general application?
> >
> > Right now application can only see "known bad" or "indeterminate"
> > there is no way to no which packets are good. Since good is the
> > desired/expected case, software has to checksum every packet.
> >
> > >
> > > I think the "2)" also requires a csum_start + csum_offset in mbuf
> > > structure, right?
> >
> > Not really, it would mean having a way to get the raw one's complement
> > sum out of the hardware. This is a good way to support the tunnel
> > protocol du jour without having to have firmware support.
> > Unfortunately, most hardware vendors don't believe in doing it that way.
> 
> It might be a good feature to have, but if most HW vendors don't support it
> why to bother?
> 
> >
> >
> > > Do you also suggest to drop IP checksum flags?
> >
> > IP checksum offload is mostly useless. If application needs to look at
> > IP, it can do whole checksum in very few instructions, the whole
> > header is in the same cache line as src/dst so the HW offload is really no
> help.
> >
[Sugesh] The checksum offload can boost the tunneling performance in OVS.
I guess the IP checksum also important as L4. In some cases, UDP checksum is
zero and no need to validate it. But Ip checksum is present on all the packets and that must be
validated all  the time. At higher packet rate, the ip checksum offload can offer slight 
performance improvement. What do you think??

> > >
> > > Will it be possible to manage tunnel checksums?
> > >
> > > I think this would be a pretty big change. If there is no additional
> > > argument than beeing more compatible with virtio/linux, I'm
> > > wondering if it's worth breaking the API. Let's wait for other opinions.
> 
> I think that what Olivier proposed is good enough and definitely a step
> forward from what we have right now.
> 
> Konstantin
> 
> > >
> > > Thanks for your feedback.
> > > Olivier