RE: [PATCH v5] event/dlb2: add support for single 512B write of 4 QEs

["McDaniel, Timothy" <timothy.mcdaniel@intel.com>]

> -----Original Message-----
> From: Richardson, Bruce <bruce.richardson@intel.com>
> Sent: Friday, June 10, 2022 10:43 AM
> To: McDaniel, Timothy <timothy.mcdaniel@intel.com>
> Cc: jerinj@marvell.com; dev@dpdk.org; Wires, Kent <kent.wires@intel.com>
> Subject: Re: [PATCH v5] event/dlb2: add support for single 512B write of 4 QEs
> 
> On Fri, Jun 10, 2022 at 03:41:00PM +0100, McDaniel, Timothy wrote:
> >
> >
> > > -----Original Message-----
> > > From: Richardson, Bruce <bruce.richardson@intel.com>
> > > Sent: Friday, June 10, 2022 8:12 AM
> > > To: McDaniel, Timothy <timothy.mcdaniel@intel.com>
> > > Cc: jerinj@marvell.com; dev@dpdk.org; Wires, Kent <kent.wires@intel.com>
> > > Subject: Re: [PATCH v5] event/dlb2: add support for single 512B write of 4
> QEs
> > >
> > > On Fri, Jun 10, 2022 at 07:35:44AM -0500, Timothy McDaniel wrote:
> > > > On Xeon, 512b accesses are available, so movdir64 instruction is able to
> > > > perform 512b read and write to DLB producer port. In order for movdir64
> > > > to be able to pull its data from store buffers (store-buffer-forwarding)
> > > > (before actual write), data should be in single 512b write format.
> > > > This commit add change when code is built for Xeon with 512b AVX support
> > > > to make single 512b write of all 4 QEs instead of 4x64b writes.
> > > >
> > > > Signed-off-by: Timothy McDaniel <timothy.mcdaniel@intel.com>
> > > > Acked-by: Kent Wires <kent.wires@intel.com>
> > > > ===
> > > >
> > > > Changes since V4:
> > > > 1) Add build-time control for avx512 support to meson.buildi, based
> > > > on implementation found in lib/acl/meson.build
> > > > 2) Add rte_vect_get_max_simd_bitwidth runtime check before using
> > > > avx512 instructions
> > > >
> > >
> > > Thanks, these changes look better for runtime support. Some further more
> > > minor comments inline below.
> > >
> > > /Bruce
> > >
> > > > Changes since V3:
> > > > 1) Renamed dlb2_noavx512.c to dlb2_sve.c, and fixed up meson.build
> > > > for new file name.
> > > >
> > > > Changes since V1:
> > > > 1) Split out dlb2_event_build_hcws into two implementations, one
> > > > that uses AVX512 instructions, and one that does not. Each implementation
> > > > is in its own source file in order to avoid build errors if the compiler
> > > > does not support the newer AVX512 instructions.
> > > > 2) Update meson.build to and pull in appropriate source file based on
> > > > whether the compiler supports AVX512VL
> > > > 3) Check if target supports AVX512VL, and use appropriate implementation
> > > > based on this runtime check.
> > > > ---
> > > >  drivers/event/dlb2/dlb2.c        | 208 +-----------------------
> > > >  drivers/event/dlb2/dlb2_avx512.c | 267
> > > +++++++++++++++++++++++++++++++
> > > >  drivers/event/dlb2/dlb2_priv.h   |  10 ++
> > > >  drivers/event/dlb2/dlb2_sve.c    | 219 +++++++++++++++++++++++++
> > > >  drivers/event/dlb2/meson.build   |  53 ++++++
> > > >  5 files changed, 556 insertions(+), 201 deletions(-)
> > > >  create mode 100644 drivers/event/dlb2/dlb2_avx512.c
> > > >  create mode 100644 drivers/event/dlb2/dlb2_sve.c
> > > >
> > > <snip>
> > >
> > > > diff --git a/drivers/event/dlb2/meson.build
> b/drivers/event/dlb2/meson.build
> > > > index f963589fd3..58146e8aef 100644
> > > > --- a/drivers/event/dlb2/meson.build
> > > > +++ b/drivers/event/dlb2/meson.build
> > > > @@ -19,6 +19,59 @@ sources = files(
> > > >          'dlb2_selftest.c',
> > > >  )
> > > >
> > > > +# compile AVX512 version if:
> > > > +# we are building 64-bit binary (checked above) AND binutils
> > > > +# can generate proper code
> > > > +
> > > > +if binutils_ok
> > > > +
> > > > +    # compile AVX512 version if either:
> > > > +    # a. we have AVX512 supported in minimum instruction set
> > > > +    #    baseline
> > > > +    # b. it's not minimum instruction set, but supported by
> > > > +    #    compiler
> > > > +    #
> > > > +    # in former case, just add avx512 C file to files list
> > > > +    # in latter case, compile c file to static lib, using correct
> > > > +    # compiler flags, and then have the .o file from static lib
> > > > +    # linked into main lib.
> > > > +
> > > > +    # check if all required flags already enabled (variant a).
> > > > +    dlb2_avx512_flags = ['__AVX512F__', '__AVX512VL__',
> > > > +                         '__AVX512CD__', '__AVX512BW__']
> > >
> > > Minor nit: are all 4 of these really necessary? I see the runtime portion
> > > only seems to check for VL?
> > >
> >
> > I will update to check for just VL
> >
> > > > +
> > > > +    dlb2_avx512_on = true
> > > > +    foreach f:dlb2_avx512_flags
> > > > +
> > > > +        if cc.get_define(f, args: machine_args) == ''
> > > > +            dlb2_avx512_on = false
> > > > +        endif
> > > > +    endforeach
> > > > +
> > > > +    if dlb2_avx512_on == true
> > > > +
> > > > +        sources += files('dlb2_avx512.c')
> > > > +        cflags += '-DCC_AVX512_SUPPORT'
> > > > +
> > > > +    elif cc.has_multi_arguments('-mavx512f', '-mavx512vl',
> > > > +                                '-mavx512cd', '-mavx512bw')
> > > > +
> > > > +        cflags += '-DCC_AVX512_SUPPORT'
> > > > +        avx512_tmplib = static_library('avx512_tmp',
> > > > +                               'dlb2_avx512.c',
> > > > +                          dependencies: [static_rte_eal,
> > > > +                                         static_rte_eventdev],
> > > > +                               c_args: cflags +
> > > > +                                       ['-mavx512f', '-mavx512vl',
> > > > +                                        '-mavx512cd', '-mavx512bw'])
> > > > +        objs += avx512_tmplib.extract_objects('dlb2_avx512.c')
> > > > +    else
> > > > +        sources += files('dlb2_sve.c')
> > > > +    endif
> > > > +else
> > > > +        sources += files('dlb2_sve.c')
> > >
> > > Since this is x86 only, do you mean SSE rather than SVE?
> > >
> > > Also, rather than adding this in the "else" legs, does the SSE version not
> > > need to always be compiled in? If the build takes the second leg, i.e.
> > > build is not mandating AVX-512, but supports it if not available, is the
> > > SSE code path not necessary for the case where the runtime machine does
> not
> > > support AVX-512?
> > >
> >
> > I'll update the name, but it's an "either or" situation. They cannot both be built
> > as currently coded.
> >
> If only the AVX-512 path is built, what happens when the runtime check for
> AVX-512 fails? Is there a scalar path that is used as fallback?
> 
> /Bruce

The file dlb2_avx512.c contains a runtime check that controls whether avx512 instructions are used. 
That file contains both implementations. The dlb2_sse.c file contains only the original/sse implementation.

Thanks,
Tim