Re: [Xen-devel] ip/udp checksum offload from minios guest

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 29 Mar 2011, at 12:37, Ian Campbell wrote:

> On Tue, 2011-03-29 at 17:17 +0100, Anil Madhavapeddy wrote:
>> I'm just adding checksum offload support into a custom guest, and wanted to 
>> clarify a few things.
>> 
>> In netfront, I can mark outgoing frames with:
>> - NETTXF_csum_blank
>> - NETTXF_data_validated
>> 
>> These refer to UDP or TCP checksums, and not the IP checksum, right?
>> Linux seems to never offload IP header checksumming, so it must be
>> offloading the UDP/TCP calculation and then adjusting the IPv4
>> checksum based on that calculation.
>> 
>> For outgoing UDP, my guest is setting the checksum to 0 (as it's
>> optional in the protocol), and calculating the full IPv4 checksum in
>> software, and all works (slowly).  However, setting NETTXF_csum_blank
>> in the outgoing frame and leaving the IPv4 checksum at 0 doesn't seem
>> to result in any adjustment by netback, and the packet gets dropped. 
> 
> I think you need to set the checksum to the psuedo-header checksum
> rather than 0 since that is what csum_blank means (such a well named
> flag!)...
> 
> It's not clear why anything would drop a UDP frame with checksum==0
> since, as you say, it is optional. My guess is that since the
> NETTXF_csum_blank and data_validated turn into an skb->ip_summed ==
> SKB_PARTIAL on the backend side this causes the Linux network stack to
> drop it because that statement conflicts with the checksum being 0.

Right, but I'm setting the IPv4 checksum to 0 too, since I want to offload the 
whole lot and never calculate a body checksum in the guest.  But if the UDP 
checksum is set to 0, then the backend can't just adjust it to obtain the IPv4 
checksum and has to iterate over the packet body at some stage.

I've tried setting the UDP checksum to 0, the pseudoheader, and the full 
checksum, and the IPv4 checksum is never set by the backend in any of these 
cases.  If I set the IPv4 checksum in software to the correct value, then 
packets go through, but the UDP checksums are never altered.

Guess it's time to start slapping printfs all over the dom0 kernel to see 
what's going on unless you know what I "should" be setting both the IPv4/UDP 
checksums to with NETTXF_csum_blank...

Anil


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