Re: [MirageOS-devel] TCP checksum offload with Xen

[Anil Madhavapeddy <anil@xxxxxxxxxx>]

On 19 Aug 2014, at 09:32, Thomas Leonard <talex5@xxxxxxxxx> wrote:

> On 19 August 2014 11:18, Anil Madhavapeddy <anil@xxxxxxxxxx> wrote:
>> 
>> On 19 Aug 2014, at 05:08, Thomas Leonard <talex5@xxxxxxxxx> wrote:
>> 
>>> I see Xen allows guests to indicate that the packet checksum is blank
>>> (XEN_NETTXF_csum_blank). Then dom0 will either fill it in itself or
>>> get the NIC to do it. How can I use this with Mirage?
>>> 
>>> I tried using ~flags:3 (Checksum_blank + Data_validated) in
>>> mirage-net-xen's netif.ml, but I don't see any TCP packets at all this
>>> way, even when the checksum is actually still being set. DHCP still
>>> works, though.
>>> 
>>> Does anyone know how this is supposed to work?
>> 
>> Probably best to trace through what netback is doing here:
>> http://lxr.free-electrons.com/source/drivers/net/xen-netback/netback.c#L1580
>> 
>> Looks like XEN_NETTXF_data_validated may need to be set as well
>> (and local packets are treated differently from off-host ones, to
>> add an extra twist).
> 
> The problem turned out to be that I was setting the flag for all
> packets, including ARPs.
> Getting this to work properly will require so API changes, because
> NETWORK needs to tell the ethernet driver which types of packet can be
> offloaded, and let the driver say which packets require checksums to
> be added.
> 
> Then similar changes are needed to ETHIF. Here are the hacks I used to test 
> it:
> 
> https://github.com/talex5/mirage/commits/csum-offload
> https://github.com/talex5/mirage-net-xen/commits/csum-offload
> https://github.com/talex5/mirage-tcpip/commits/csum-offload

Looks good -- since V2.mli isn't tagged yet, this can just extend the call
to `write` instead of having a `write2`.  However, would it be better to
define an interface flags type in `NETWORK`, and just let the `write` call
supply a list of flags?  That would permit easier extension to other
offloads in the future.

A pull request to mirage/mirage of the WIP patches would be good to track
this in the issue tracker.

cheers,
Anil
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