Re: [dpdk-dev] [PATCH 1/5] vmxnet3: Fix VLAN Rx stripping

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Oct 28, 2014 at 09:57:14PM +0000, Yong Wang wrote:
> On 10/22/14, 6:39 AM, "Stephen Hemminger" <stephen@networkplumber.org>
> wrote:
> 
> 
> >On Mon, 13 Oct 2014 18:42:18 +0000
> >Yong Wang <yongwang@vmware.com> wrote:
> >
> >> Are you referring to the patch as a whole or your comment is about the
> >>reset of vlan_tci on the "else" (no vlan tags stripped) path?  I am not
> >>sure I get your comments here.  This patch simply fixes a bug on the rx
> >>vlan stripping path (where valid vlan_tci stripped is overwritten
> >>unconditionally later on the rx path in the original vmxnet3 pmd
> >>driver). All the other pmd drivers are doing the same thing in terms of
> >>translating descriptor status to rte_mbuf flags for vlan stripping.
> >
> >I was thinking that there are many fields in a pktmbuf and rather than
> >individually
> >setting them (like tci). The code should call the common
> >rte_pktmbuf_reset before setting
> >the fields.  That way when someone adds a field to mbuf they don't have
> >to chasing
> >through every driver that does it's own initialization.
> 
> Currently rte_pktmbuf_reset() is used in rte_pktmbuf_alloc() but looks
> like most pmd drivers use rte_rxmbuf_alloc() to replenish rx buffers,
> which directly calls __rte_mbuf_raw_alloc
> () without calling rte_pktmbuf_reset(). How about we change that in a
> separate patch to all pmd drivers so that we can keep their behavior
> consistent?
> 

We can look to do that, but we need to beware of performance regressions if 
we do so. Certainly the vector implementation of the ixgbe would be severely 
impacted performance-wise if such a change were made. However, code paths 
which are not as highly tuned, or which do not need to be as highly tuned 
could perhaps use the standard function.

The main reason for this regression is that reset will clear all fields of 
the mbuf, which would be wasted cycles for a number of the PMDs as they will 
later set some of the fields based on values in the receive descriptor.  
Basically, on descriptor rearm in a PMD, the only fields that need to be 
reset would be those not set by the copy of data from the descriptor.

/Bruce