Re: [dpdk-dev] [PATCH v1 5/5] ixgbe: Add LRO support

[Vlad Zolotarov <vladz@cloudius-systems.com>]

On 03/04/15 02:33, Stephen Hemminger wrote:
> On Tue,  3 Mar 2015 21:48:43 +0200
> Vlad Zolotarov <vladz@cloudius-systems.com> wrote:
>
>> +	next_desc:
>> +		/*
>> +		 * The code in this whole file uses the volatile pointer to
>> +		 * ensure the read ordering of the status and the rest of the
>> +		 * descriptor fields (on the compiler level only!!!). This is so
>> +		 * UGLY - why not to just use the compiler barrier instead? DPDK
>> +		 * even has the rte_compiler_barrier() for that.
>> +		 *
>> +		 * But most importantly this is just wrong because this doesn't
>> +		 * ensure memory ordering in a general case at all. For
>> +		 * instance, DPDK is supposed to work on Power CPUs where
>> +		 * compiler barrier may just not be enough!
>> +		 *
>> +		 * I tried to write only this function properly to have a
>> +		 * starting point (as a part of an LRO/RSC series) but the
>> +		 * compiler cursed at me when I tried to cast away the
>> +		 * "volatile" from rx_ring (yes, it's volatile too!!!). So, I'm
>> +		 * keeping it the way it is for now.
>> +		 *
>> +		 * The code in this file is broken in so many other places and
>> +		 * will just not work on a big endian CPU anyway therefore the
>> +		 * lines below will have to be revisited together with the rest
>> +		 * of the ixgbe PMD.
>> +		 *
>> +		 * TODO:
>> +		 *    - Get rid of "volatile" crap and let the compiler do its
>> +		 *      job.
>> +		 *    - Use the proper memory barrier (rte_rmb()) to ensure the
>> +		 *      memory ordering below.
> This comment screams "this is broken".
> Why not get proper architecture independent barriers in DPDK first.

This series is orthogonal to the issue above. I just couldn't stand to 
mention this ugliness when I noticed it on the way.
Note that although this is obviously not the right way to write this 
kind of code it is still not a bug and most likely the performance 
implications are minimal here.
The only overhead is that there may be read "too much" data from the 
descriptor that we may not actually need. The descriptor is 16 bytes so 
this doesn't seem to be a critical issue.

So, fixing the above issue may wait, especially since the same s..t may 
be found in other Intel PMDs (see i40e for example). Fixing this issue 
should be a matter of a massive cleanup series that cover all the 
relevant PMDs. Of course we may start with ixgbe but even in this single 
PMD there are at least 3 non-LRO related functions that have to be 
fixed, so IMHO even fixing ONLY ixgbe should be a matter of a separate 
series.