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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2014-10-29 09:04, Bruce Richardson:
> 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.

This is typically a trade-off situation.
I think that we should prefer the performance.

-- 
Thomas