Re: [dpdk-dev] [RFC PATCH v2] build: add platform meson option

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Feb 23, 2021 at 08:45:09AM +0000, Juraj Linkeš wrote:
> 
> 
> > -----Original Message-----
> > From: David Christensen <drc@linux.vnet.ibm.com>
> > Sent: Monday, February 22, 2021 10:25 PM
> > To: Juraj Linkeš <juraj.linkes@pantheon.tech>; Bruce Richardson
> > <bruce.richardson@intel.com>
> > Cc: thomas@monjalon.net; Honnappa.Nagarahalli@arm.com; dev@dpdk.org
> > Subject: Re: [RFC PATCH v2] build: add platform meson option
> > 
> > 
> > 
> > On 2/19/21 1:11 AM, Juraj Linkeš wrote:
> > >
> > >
> > >> -----Original Message-----
> > >> From: Bruce Richardson <bruce.richardson@intel.com>
> > >> Sent: Wednesday, January 6, 2021 3:43 PM
> > >> To: David Christensen <drc@linux.vnet.ibm.com>
> > >> Cc: Juraj Linkeš <juraj.linkes@pantheon.tech>; thomas@monjalon.net;
> > >> Honnappa.Nagarahalli@arm.com; dev@dpdk.org
> > >> Subject: Re: [RFC PATCH v2] build: add platform meson option
> > >>
> > >> On Tue, Jan 05, 2021 at 02:17:44PM -0800, David Christensen wrote:
> > >>>>> The current meson option 'machine' should only specify the ISA,
> > >>>>> which is not sufficient for Arm, where setting ISA implies other
> > >>>>> setting as
> > >> well.
> > >>>>> Add a new meson option, 'platform', which differentiates the type
> > >>>>> of the build
> > >>>>> (native/generic) and sets machine accordingly, unless the user
> > >>>>> chooses to override it.
> > >>>>> The 'machine' option also doesn't describe very well what it sets,
> > >>>>> so introduce a new option 'cpu_instruction_set', but keep
> > >>>>> 'machine' for backward compatibility.
> > >>>>> These two new variables, taken together, achieve what 'machine'
> > >>>>> was setting per architecture - setting the ISA in x86 build and
> > >>>>> setting native/default 'build type' in aarch64 build - is now
> > >>>>> properly being set for all architectures in a uniform manner.
> > >>>>>
> > >>>>> Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
> > >>>>> ---
> > >>>>>    config/arm/meson.build        |  4 +--
> > >>>>>    config/meson.build            | 47 +++++++++++++++++++++++++----------
> > >>>>>    devtools/test-meson-builds.sh |  9 ++++---
> > >>>>>    meson_options.txt             |  8 ++++--
> > >>>>>    4 files changed, 47 insertions(+), 21 deletions(-)
> > >>>>>
> > >>>>> diff --git a/config/arm/meson.build b/config/arm/meson.build index
> > >>>>> 42b4e43c7..6b09a74a7 100644
> > >>>>> --- a/config/arm/meson.build
> > >>>>> +++ b/config/arm/meson.build
> > >>>>> @@ -3,10 +3,10 @@
> > >>>>>    # Copyright(c) 2017 Cavium, Inc
> > >>>>>
> > >>>>>    # for checking defines we need to use the correct compiler
> > >>>>> flags -march_opt = '-
> > >>>>> march=@0@'.format(machine)
> > >>>>> +march_opt = '-march=@0@'.format(cpu_instruction_set)
> > >>>>>
> > >>>>>    arm_force_native_march = false
> > >>>>> -arm_force_default_march = (machine == 'default')
> > >>>>> +arm_force_default_march = (platform == 'generic')
> > >>>>>
> > >>>>>    flags_common_default = [
> > >>>>>    	# Accelarate rte_memcpy. Be sure to run unit test
> > >>>>> (memcpy_perf_autotest) diff --git a/config/meson.build
> > >>>>> b/config/meson.build index a3154e29c..647116513 100644
> > >>>>> --- a/config/meson.build
> > >>>>> +++ b/config/meson.build
> > >>>>> @@ -63,42 +63,63 @@ if not is_windows
> > >>>>>    			pmd_subdir_opt)
> > >>>>>    endif
> > >>>>>
> > >>>>> -# set the machine type and cflags for it
> > >>>>> +platform = get_option('platform')
> > >>>>> +
> > >>>>> +# set the cpu_instruction_set type and cflags for it
> > >>>>>    if meson.is_cross_build()
> > >>>>> -	machine = host_machine.cpu()
> > >>>>> +	cpu_instruction_set = host_machine.cpu()
> > >>>>>    else
> > >>>>> -	machine = get_option('machine')
> > >>>>> +	cpu_instruction_set = get_option('cpu_instruction_set')
> > >>>>> +	if get_option('machine') != 'auto'
> > >>>>> +		warning('The "machine" option is deprecated. ' +
> > >>>>> +		        'Please use "cpu_instruction_set" instead.')
> > >>>>> +		if cpu_instruction_set != 'auto'
> > >>>>> +			error('Setting both "machine" and ' +
> > >>>>> +			      '"cpu_instruction_set" is unsupported.')
> > >>>>> +		endif
> > >>>>> +		cpu_instruction_set = get_option('machine')
> > >>>>> +	endif
> > >>>>> +endif
> > >>>>> +
> > >>>>> +if platform == 'native'
> > >>>>> +	if cpu_instruction_set == 'auto'
> > >>>>> +		cpu_instruction_set = 'native'
> > >>>>> +	endif
> > >>>>> +elif platform == 'generic'
> > >>>>> +	if cpu_instruction_set == 'auto'
> > >>>>> +		cpu_instruction_set = 'default'
> > >>>>> +	endif
> > >>>>>    endif
> > >>>>>
> > >>>>> -# machine type 'default' is special, it defaults to the per arch
> > >>>>> agreed common
> > >>>>> +if cpu_instruction_set == 'default'
> > >>>>> +# cpu_instruction_set type 'default' is special, it defaults to
> > >>>>> +the per arch agreed common
> > >>>>>    # minimal baseline needed for DPDK.
> > >>>>>    # That might not be the most optimized, but the most portable
> > >>>>> version while  # still being able to support the CPU features
> > >>>>> required for
> > >> DPDK.
> > >>>>>    # This can be bumped up by the DPDK project, but it can never
> > >>>>> be an  # invariant like 'native'
> > >>>>> -if machine == 'default'
> > >>>>>    	if host_machine.cpu_family().startswith('x86')
> > >>>>>    		# matches the old pre-meson build systems default
> > >>>>> -		machine = 'corei7'
> > >>>>> +		cpu_instruction_set = 'corei7'
> > >>>>>    	elif host_machine.cpu_family().startswith('arm')
> > >>>>> -		machine = 'armv7-a'
> > >>>>> +		cpu_instruction_set = 'armv7-a'
> > >>>>>    	elif host_machine.cpu_family().startswith('aarch')
> > >>>>>    		# arm64 manages defaults in config/arm/meson.build
> > >>>>> -		machine = 'default'
> > >>>>> +		cpu_instruction_set = 'default'
> > >>>>>    	elif host_machine.cpu_family().startswith('ppc')
> > >>>>> -		machine = 'power8'
> > >>>>> +		cpu_instruction_set = 'power8'
> > >>>>>    	endif
> > >>>>>    endif
> > >>>
> > >>> This change forces the build on a P9 system to use the P8 instruction set.
> > >>> Prior to this change the "native" machine type was used which
> > >>> resulted in P9 instructions when built on a P9 system.  How do I
> > >>> force the build to use the
> > >>> power9 instruction set in this case?
> > >>>
> > >>> Dave
> > >>
> > >>  From looking at the patch, setting the "platform" to "native", or
> > >> the instruction_set to "native" should do this.
> > >> While I consider generic builds a good thing, I wonder if there is an
> > >> expectation that "native" is always the default build type for DPDK builds?
> > >>
> > >> /Bruce
> > >
> > > I left this patch alone so that people could chime in, but noone did, so let's try
> > to find some agreeable solution.
> > >
> > > My current thoughts are as such:
> > > The platform parameter specifies a set of DPDK options that will be used. This
> > is what arm uses for its builds, x86 and ppc don't use this.
> > > The cpu_instruction_set sets just one configuration among the "platform"
> > configuration set.
> > > We want the build to work on most machines of the machine architecture.
> > That implies platform=generic (as in use config options that will work on
> > everything of that architecture) and cpu_instruction_set=generic (as in use ISA
> > that will work on all cpus of the build machine architecture).
> > > Setting cpu_instruction_set=generic changes the build without cmdline options
> > for ppc. Thus, the expectation may be that cpu_instruction_set should be native
> > by default.
> > >
> > > For arm, cpu_instruction_set is ignored (and thus the value doen't matter),
> > since we can't use that without other config options (e.g. DPDK config for an
> > SoC (such as RTE_ARM_FEATURE_ATOMICS) used with an invalid
> > cpu_instuction_set). That means the only relevant parameter for Arm is platform
> > and if we want to have a build usable on most machines of the build type, we
> > have to use platform=generic.
> > >
> > > For x86 and ppc, there's no difference between native and generic platform (as
> > it's a new argument, the purpose of which is to differentiate DPDK config across
> > platforms, which doesn't exist for x86 and ppc - DPDK config is the same (correct
> > me if I'm wrong)).
> > >
> > > So it basically boils down to what should be the default value of
> > cpu_instruction_set when platform=generic (for platform=native, it's obviously
> > native):
> > > 1. cpu_instruction_set=native, this would preserve the current behavior, but
> > we won't use the 'auto' default. I think we can fall back to this if we don't agree
> > on anything better.
> > > 2. cpu_instruction_set=auto, the same as cpu_instruction_set=generic,
> > > 3. cpu_instruction_set=generic, this changes behavior for ppc builds, but we
> > may be able to remedy this:
> > > Similarly to arm (arm is using platform for this, but the idea is the same), if
> > cpu_instruction_set is generic, we can do some discovery for pcc and set the ISA
> > accordingly (either power8 or power9). If I understand it correctly, power8 is a
> > different architecture from power9 (I could be wrong on this), so this is
> > desirable. There's some logic in config/ppc/meson.build, but it doesn't seem
> > sufficient as a discovery mechanism between power8/power9.
> > 
> > POWER8 code (Power ISA v2.07) runs on POWER9 CPUs (Power ISA v3.0), so
> > setting cpu_instruction_set to GENERIC is a reasonable option. POWER10 CPUs
> > (Power ISA v3.1) are around the corner so I want to make sure developers can
> > tune the application for the platform as easily as possible.  I'm fine with
> > supporting GENERIC, just need clear instructions on how to build for a particular
> > ISA.
> > 
> 
> That's good to hear. Setting cpu_instruction_set will set the ISA.
> 
> We'll need to document the behavior properly, but I'm not sure where - Bruce? Seems like meson_options.txt doesn't have enough room for that.
> 

Yes, agreed. I suggest putting in the option help text a link to the
documentation where we explain the options more fully.

/Bruce