[Xen-devel] Re: skb_checksum_setup() placement in pv-ops vs. legacy kernel

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 03.12.10 at 13:06, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> You mean a packet flowing pvops-domU -> pvops-dom0 -> legacy?

I was actually just thinking in terms of a simple pair (see below).

> In this case the dom0 kernel does the necessary setup at (*) in the
> pvops-domU -> (*) pvops-dom0 hop so there is nothing to do on the
> pvops-dom0 (*) ->legacy hop.
> 
> If the legacy kernel forwards the packet further it will have to do the
> setup on its egress path, this is the same if dom0 is pvops or legacy.
> 
>> Secondary question was whether the duplicated effort on transmission the 
> other way around
>> may be a (performance) issue.
> 
> You mean the legacy (*) -> (*) pvops-dom0 -> pvops-domU case?
> 
> In that case the setup is done at the two (*)'s but it is not really
> "duplicated" as such since it is in the context of two separate skbs. if
> the dom0 was legacy then the second one would still happen but on the
> egress path.
> 
> I have a feeling I'm not understanding what your concern is correctly.
> If the above isn't what you mean can you give an example of the path of
> the packet and when the setup is (not) occurring.

pv-ops-{front,back}end -> legacy-{back,front} (for example a
pv-ops DomU sending a packet to (not through) a legacy Dom0,
or pv-ops Dom0 sending to legacy DomU). Of course, if the
packet fully passes the backend domain's stack, it will have
undergone the setup at least once (either on its way into or
out of that stack).

Jan


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