On 03/13/15 13:28, Ananyev, Konstantin wrote: > Hi Olivier, > >> -----Original Message----- >> From: Olivier MATZ [mailto:olivier.matz at 6wind.com] >> Sent: Friday, March 13, 2015 9:08 AM >> To: Vlad Zolotarov; Ananyev, Konstantin; dev at dpdk.org >> Subject: Re: [dpdk-dev] [PATCH v6 3/3] ixgbe: Add LRO support >> >> Hi Vlad, >> >> On 03/11/2015 05:54 PM, Vlad Zolotarov wrote: >>>>>> About the existing RX/TX functions and PPC support: >>>>>> Note that all of them were created before PPC support for DPDK was >>>>>> introduced. >>>>>> At that moment only IA was supported. >>>>>> That's why in some places where you would expect to see 'mb()' there >>>>>> are 'volatile' and/or ' rte_compiler_barrier' instead. >>>>>> Why all that places wasn't updated when PPC support was added - >>>>>> that's another question. >>>>>> From my understanding - with current implementation some of DPDK >>>>>> PMDs RX/TX functions and rte_ring wouldn't work correctly >>>>> on PPC. >>>>>> So, I suppose we need to decide for ourselves - do we really want to >>>>>> support PPC and other architectures with non-IA memory >>>>> model or not? >>>>>> If not, then I think we don't need any mb()s inside recv_pkts_lro() >>>>>> - just rte_compiler_barrier seems enough, and no point to >>>>> complain about >>>>>> it in comments. >>>>>> If yes - then why to introduce a new function with a known potential >>>>>> bug? >>>>> In order to introduce a new function with the proper implementation or >>>>> to fix any other places with the similar weakness I would need a proper >>>>> tools like a proper platform-dependent barrier-macros similar to >>>>> smp_Xmb() Linux macros that reduce to a compiler barrier where >>>>> appropriate or to a proper memory fence where needed. >>>> I understand that. >>>> Let's add new macro for that: rte_smp_Xmb() or something, >>>> so it would be just rte_compiler_barrier() for x86 and a proper mb() >>>> for PPC. >>> There was an idea to use the C11 built-in memory barriers. I suggest we >>> open a separate discussion about that and add these and the appropriate >>> fixes in a separate series. There are quite a few places to fix anyway, >>> which are currently broken on PPC so this patch doesn't make things any >>> worse. However adding a new memory barrier doesn't belong to an LRO >>> functionality and thus to this series. >> This is an interesting discussion. Just for reference, I submitted a >> patch on this topic but it was probably too early as only Intel >> architecture was supported at that time. >> >> See http://dpdk.org/ml/archives/dev/2014-May/002597.html > I do remember that conversation :) > At that moment, as nothing except IA wasn't supported, I feel it was not > needed. > Though now, if we do want to support PPC and other architectures with weak > memory model, > I think we do need to introduce some platform dependent set of Xmb() macros. > See http://dpdk.org/ml/archives/dev/2014-October/006729.html > > Actually while thinking about it once again: > Is there any good use for rte_compiler_barrier() for PPC memory model? > I can't think about any. > So I wonder can't we just make for PPC: > #define rte_compiler_barrier rte_mb > While keeping it as it is for IA. > Would save us from searching/replacing though all the code.
I wonder why should we invent a wheel? Like Avi has proposed we may use the existing standard C library primitives for that. See this http://en.cppreference.com/w/c/atomic. I don't know what's the state of icc in this area though... ;) Pros: * Zero maintenance. * Multi-platform support. * It seems that this is the direction the industry is going to (as opposed to the discussed above mb(), rmb(), wmb() model). Cons: * The model is a bit different from what most of the kernel programmers used to. * The current code adaptation would be a bit more painful (due to first "cons"). I think this could be a very nice move. For a user space for sure. The open question is a KNI component. I don't know how much code is shared between kernel and user space DPDK code but if there isn't much - then we may still go for a built-in C atomics primitives in a user space and do whatever we choose in the KNI... > > Konstantin > > > >> Regards, >> Olivier
