[dpdk-dev] [PATCH v5] examples/l3fwd: em path performance fix

[Kulasek, TomaszX]

> -----Original Message-----
> From: Jan Viktorin [mailto:viktorin at rehivetech.com]
> Sent: Friday, March 18, 2016 12:57
> To: Thomas Monjalon <thomas.monjalon at 6wind.com>
> Cc: Jerin Jacob <jerin.jacob at caviumnetworks.com>; Kulasek, TomaszX
> <tomaszx.kulasek at intel.com>; dev at dpdk.org; jianbo.liu at linaro.org
> Subject: Re: [dpdk-dev] [PATCH v5] examples/l3fwd: em path performance fix
> 
> Hello Thomas, Jerin, Tomasz, all...
> 
> On Fri, 18 Mar 2016 12:00:24 +0100
> Thomas Monjalon <thomas.monjalon at 6wind.com> wrote:
> 
> > 2016-03-18 16:22, Jerin Jacob:
> > > On Fri, Mar 18, 2016 at 11:04:49AM +0100, Thomas Monjalon wrote:
> > > > 2016-03-18 10:52, Tomasz Kulasek:
> > > > > +#if !defined(NO_HASH_MULTI_LOOKUP) && defined(__ARM_NEON)
> > > >
> > > > I think we should use CONFIG_RTE_ARCH_ARM_NEON here.
> > > > Any ARM maintainer to confirm?
> > >
> > > __ARM_NEON should work existing GCC, but it is better to use
> > > RTE_MACHINE_CPUFLAG_NEON as -it has been generated by probing the
> > > compiler capabilities.
> > > -it's future-proof solution to support clang or other gcc versions
> > > in future
> >
> > I agree to use RTE_MACHINE_CPUFLAG_NEON.
> >
> > I just don't understand why CONFIG_RTE_ARCH_ARM_NEON has been
> introduced.
> > It seems to be used to disable NEON on ARMv7:
> 
> This is true. You should be able to disable the NEON-specific code if it
> is unwanted. Eg., the memcpy operations are not always faster with NEON.
> But...
> 
> $ git grep ARM_NEON
> ...
> lib/librte_eal/common/include/arch/arm/rte_memcpy_32.h:45:#ifdef
> __ARM_NEON_FP
> lib/librte_eal/common/include/arch/arm/rte_memcpy_32.h:328:#endif /*
> __ARM_NEON_FP */ ...
> 
> From this point of view, this is wrong and should be fixed to check a
> different constant.
> 
> > 	ifeq ($(CONFIG_RTE_ARCH_ARM_NEON),y)
> > 	MACHINE_CFLAGS += -mfpu=neon
> > 	endif
> 
> However, there is another possible way of understanding these options.
> We can (well, unlikely and I am about to say 'never') have an ARM
> processor without NEON. This cannot be detected by gcc as it does not know
> the target processor... So from my point of view:
> 
> * CONFIG_RTE_ARCH_ARM_NEON says "my CPU does (not) support NEON" or "I
>   want to enable/disable NEON" while
> * RTE_MACHINE_CPUFLAG_NEON says, the _compiler_ supports NEON
> 
> I'll send a patch trying to solve this.
> 
> Regards
> Jan

Hi

As I understand with your last patch it's safe and preferred to use RTE_MACHINE_CPUFLAG_NEON for ARM Neon detection? If so, I can include this modification for whole l3fwd in v6 of this patch.

Tomasz.