Re: [PATCH 04/13] acl: allow zero verdict

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Michal Miroslaw [mailto:mirq-linux@rere.qmqm.pl]
> Sent: Tuesday, December 13, 2016 4:14 PM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH 04/13] acl: allow zero verdict
> 
> On Tue, Dec 13, 2016 at 03:13:42PM +0000, Ananyev, Konstantin wrote:
> [...]
> > > > > > > Subject: [dpdk-dev] [PATCH 04/13] acl: allow zero verdict
> > > > > > >
> > > > > > > Signed-off-by: Michał Mirosław <michal.miroslaw@atendesoftware.pl>
> > > > > > > ---
> > > > > > >  lib/librte_acl/rte_acl.c         | 3 +--
> > > > > > >  lib/librte_acl/rte_acl.h         | 2 --
> > > > > > >  lib/librte_table/rte_table_acl.c | 2 +-
> > > > > > >  3 files changed, 2 insertions(+), 5 deletions(-)
> > > > > > >
> > > > > > > diff --git a/lib/librte_acl/rte_acl.c b/lib/librte_acl/rte_acl.c
> > > > > > > index 8b7e92c..d1f40be 100644
> > > > > > > --- a/lib/librte_acl/rte_acl.c
> > > > > > > +++ b/lib/librte_acl/rte_acl.c
> > > > > > > @@ -313,8 +313,7 @@ acl_check_rule(const struct rte_acl_rule_data *rd)
> > > > > > >  	if ((RTE_LEN2MASK(RTE_ACL_MAX_CATEGORIES, typeof(rd->category_mask)) &
> > > > > > >  			rd->category_mask) == 0 ||
> > > > > > >  			rd->priority > RTE_ACL_MAX_PRIORITY ||
> > > > > > > -			rd->priority < RTE_ACL_MIN_PRIORITY ||
> > > > > > > -			rd->userdata == RTE_ACL_INVALID_USERDATA)
> > > > > > > +			rd->priority < RTE_ACL_MIN_PRIORITY)
> > > > > > >  		return -EINVAL;
> > > > > > >  	return 0;
> > > > > > >  }
> > > > > >
> > > > > > I am not sure, how it supposed to work properly?
> > > > > > Zero value is reserved and ifnicates that no match were found for that input.
> > > > >
> > > > > This is actually in use by us. In our use we don't need to differentiate
> > > > > matching a rule with zero verdict vs not matching a rule at all. I also
> > > > > have a patch that changes the value returned in non-matching case, but
> > > > > it's in "dirty hack" state, as of yet.
> > > >
> > > > With that chane rte_acl_classify() might produce invalid results.
> > > > Even if you don't need it (I still don't understand how) , it doesn't mean other people
> > > > don't  need it either and it is ok to change it.
> > > >
> > > > >
> > > > > The ACL code does not treat zero userdata specially, so this is only
> > > > > a policy choice and as such would be better to be made by the user.
> > > >
> > > > I believe it does.
> > > > userdata==0 is a reserved value.
> > > > When rte_acl_clasify() returns 0 for that particular input, it means 'no matches were found'.
> > >
> > > Dear Konstantin,
> > >
> > > Can you describe how the ACL code treats zero specially? I could not find
> > > anything, really. The only thing I found is that iff I use zero userdata
> > > in a rule I won't be able to differentiate a case where it matched from
> > > a case where no rule matched.
> >
> > Yes, that's what I am talking about.
> >
> > > If I all my rules have non-zero userdata,
> > > then this patch changes nothing.
> >
> > Ok, then why do you remove a code that does checking for invalid userdata==0?
> > That supposed to prevent user to setup invalid value by mistake.
> >
> >  But if I have a table where 0 means drop
> > > (default-drop policy) then being able to use zero userdata in DROP rules
> > > makes the ACLs just that more useful.
> >
> > Ok, and what prevents you from do +1 to your policy values before
> > you insert it into the ACL table and -1 after you retrieved it via rte_acl_classify()?
> 
> The check is enforcing an assumption that all users want to distinguish
> the cases whether any rule matched and whether no rules matched. Not all
> users do, hence the assumption is invalid and this patch removes it.

The check is based on the assumption that users might need to distinguish
the situation when no rules were matched.
To support that we need a reserved userdata value, which would mean
NO_MATCH.
>From what I heard, most users do need this ability, those who don't
can easily overcome it.
> 
> Yes, people can work around it by loosing 1 of 2^32 useful values and
> convoluting their code.

Yes, one of 2^32 values is reserved.
Any reason why (2^32 - 1) values might not be enough?

> 
> You seem to argue that 0 is somehow an invalid value, but I can't find
> anything in the ACL that would require it to be so. Could you point me
> to the code in DPDK where this actually matters?

It was a while, when I looked into ACL code in details, but as remember
that's the only reason: we need some value to be reserved as NO_MATCH.
Let say in build_trie() we set results to zero for rules with unused categories:
for (m = context->cfg.num_categories; 0 != m--; ) {
                        if (rule->f->data.category_mask & (1 << m)) {
                                end->mrt->results[m] = rule->f->data.userdata;
                                end->mrt->priority[m] = rule->f->data.priority;
                        } else {
                                end->mrt->results[m] = 0;
                                end->mrt->priority[m] = 0;
                        }
                }

Konstantin