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

[Yong Wang <yongwang@vmware.com>]

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.

Yong

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.
>
>-- 
>Thomas