Re: Performance enhancements: Checksum offloads.

["Durrant, Paul" <xadimgnik@xxxxxxxxx>]

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.
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.
  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.