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

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

2014-10-29 17:57, Yong Wang:
> Sounds good to me but it does look like the rte_rxmbuf_alloc() could use
> some comments to make it explicit that rte_pktmbuf_reset() is avoided by
> design for the reasons that Bruce described.  Furthermore,
> rte_rxmbuf_alloc() is duplicated in almost all the pmd drivers.  Will it
> make sense to promote it to a public API?  Just a thought.

Yes, it makes sense.


> On 10/29/14, 2:41 AM, "Thomas Monjalon" <thomas.monjalon@6wind.com> wrote:
> 
> >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.