Re: [PATCH v4] event/dlb2: add support for single 512B write of 4 QEs

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

On Mon, May 23, 2022 at 11:09:55AM -0500, Timothy McDaniel wrote:
> On Xeon, as 512b accesses are available, 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 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        | 206 +-----------------------
>  drivers/event/dlb2/dlb2_avx512.c | 267 +++++++++++++++++++++++++++++++
>  drivers/event/dlb2/dlb2_priv.h   |   8 +
>  drivers/event/dlb2/dlb2_sve.c    | 219 +++++++++++++++++++++++++
>  drivers/event/dlb2/meson.build   |  14 ++
>  5 files changed, 513 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..0ad4d31785 100644
> --- a/drivers/event/dlb2/meson.build
> +++ b/drivers/event/dlb2/meson.build
> @@ -19,6 +19,20 @@ sources = files(
>          'dlb2_selftest.c',
>  )
>  
> +dlb2_avx512_support = false
> +
> +if dpdk_conf.has('RTE_ARCH_X86_64')
> +        dlb2_avx512_support = (
> +	    cc.get_define('__AVX512VL__', args: machine_args) != ''
> +	)
> +endif
> +
> +if dlb2_avx512_support == true
> +        sources += files('dlb2_avx512.c')
> +else
> +        sources += files('dlb2_sve.c')
> +endif
> +
>  headers = files('rte_pmd_dlb2.h')
>  
>  deps += ['mbuf', 'mempool', 'ring', 'pci', 'bus_pci']

I believe this can be improved upon further, since it still does not allow
a generic build to opportunistically use the AVX-512 code path. It also
makes the runtime check largely pointless as the whole build will have been
done with global AVX-512 support, meaning that the binary likely will fail
to run if AVX-512 is not available.

Instead, I'd recommend doing as other places in DPDK - such as in ACL
library, or i40e or ice net drivers - where we not only check the current
build support, but also check the compiler support. That way, even if we
are building for e.g. a target of AVX2, we can still build the AVX-512
parts using the appropriate compiler flags, and choose them
opportunistically at runtime. See the meson.build files in any of the above
component directories for examples.

Regards,

/Bruce