Re: Performance enhancements: Checksum offloads.

[Edwin Torok <edvin.torok@xxxxxxxxxx>]

> On 7 Jun 2022, at 11:24, Durrant, Paul <xadimgnik@xxxxxxxxx> wrote:
> 
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> On 07/06/2022 10:32, Martin Harvey wrote:
>> Since I've been working on a performance related ticket today, a thought 
>> about future enhancements:
>> - Can we do checksum offloads all the way from H/W to the guest - if the 
>> dom0 drivers support the offloads, then there should be a way to get the 
>> calculated checksums all the way thru to the guest?
> 
> Absolutely; the netif protocol has support for offloading (see 
> NETRXF_data_validated) and you can set the 'Correct TCP/UDP Checksum Value' 
> switch to 0 in the XENNET advanced properties if you want to avoid checksum 
> recalculation.

Thanks, we should probably do some internal benchmarks with that setting. At 
least users with well-behaved applications could chose to avoid the perf hit.

Last time I looked at this there was some extra checksum recalculation going on 
when splitting up large receive offloaded packets (which are IIRC around ~64k 
in size, much larger than even jumbo frames) and those had to get a checksum 
from somewhere. If we can tell it "its fine, it has a valid checksum" without 
actually computing one then we can avoid all that, I assume the above setting 
will avoid recomputation in this case too?

> 
> The reason checksum recalculation is that validated packets coming from 
> netback do not always have the correct checksum. Specifically packets from 
> one VM to another VM on the same host may only contain a pseudo-header 
> checksum. Also, I recall some NIC drivers zero-ing the checksum during 
> validation. Now, you'd expect that when XENNET says to the network stack 'all 
> is good with this packet' that nothing further up the stack should care about 
> the checksum value however some of the good ole Citrix ICA stack (for 
> example) actually does... so that's why we have to take the performance hit 
> and fully recalculate. It's all rather silly.

That would probably be broken when run on bare metal with NIC drivers that 
support hardware offload too though?
Perhaps there could be a setting to zero the checksum field when its 
computation is offloaded to more easily detect these cases, at least as 
something to use during debugging / automated testing?

Best regards,
--Edwin

> 
>  Paul
> 
>> - This could require some thought, and I'm not sure what the various 
>> security implications are.
>> - Pretty sure it would provide a useful performance boost.
>> MH.
>