On Thu, Apr 14, 2016 at 11:09 AM, Prathap T <prathap....@gmail.com> wrote:
> Hi OVS-Team:
>
> We are porting OVS onto one of our hardware and I had a question in the
> kernel datapath implementation.
>
> In the function - masked_flow_lookup
>
> ovs_flow_mask_key(&masked_key, unmasked, mask);
> hash = flow_hash(&masked_key, &mask->range);
> head = find_bucket(ti, hash);
> (*n_mask_hit)++;
> hlist_for_each_entry_rcu(flow, head, flow_table.node[ti->node_ver]) {
> if (flow->mask == mask && flow->flow_table.hash == hash &&
> flow_cmp_masked_key(flow, &masked_key, &mask->range))
> return flow;
> }
>
> why is that the code compares the flow->mask == mask; is it not enough to
> just compare the key?? Is it just an optimization??
> If mask comparison is needed, can you please explain the traffic context
> that makes this necessary?
It's not an optimization, it's necessary for correctness.
Masks are inherently tied to the flows that they are masking. The
simplest implementation of what is being done here would be a linear
search over each flow, masking the incoming packet headers and doing a
comparison with the stored flow. As an optimization, we do the masking
once for each mask and then do a hash table lookup over all flows that
use that mask since it is common for flows to share masks. However, we
still must act as if the mask and flow are paired.
Imagine a 1 bit long flow. The flow in the table has the value 0 and
the mask is 1 (significant). There is also another mask with the value
0 (not significant). If a packet came in with the header value of 1
and it tried the second mask first then the value would become 0,
matching the flow in table on the basis of a strict comparison.
However, in reality these flows don't match.
> The reason I'm asking is that, we have some space issues and cannot store
> anymore than the keys in the hardware structure. If the mask comparison is
> not needed, we may want to skip it. If it is needed, we will have to
> compromise on some of the key elements.
I suspect that there are better ways of using your hardware rather
than directly porting this logic. For example, this function is pretty
much exactly mirroring the logic implemented in a TCAM.
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