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

["Juraj Linkeš" <juraj.linkes@pantheon.tech>]

> -----Original Message-----
> From: Bruce Richardson <bruce.richardson@intel.com>
> Sent: Tuesday, February 23, 2021 10:43 AM
> To: Juraj Linkeš <juraj.linkes@pantheon.tech>
> Cc: David Christensen <drc@linux.vnet.ibm.com>; thomas@monjalon.net;
> Honnappa.Nagarahalli@arm.com; dev@dpdk.org
> Subject: Re: [RFC PATCH v2] build: add platform meson option
> 
> 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

You mentioned in the kni cross compile patch that we should do a separate patch where we explain the option in docs and I believe this would go into that patch. Before we do that, let's figure out the best description without the link. I currently have:
option('cpu_instruction_set', type: 'string', value: 'auto',
	description: 'Set the target machine ISA (instruction set architecture). Will be set according to the platform option by default.')
option('platform', type: 'string', value: 'generic',
	description: 'Platform to build, either "native" or "generic".')

What do you think? Should we expand the platform description a bit?