On Wed, Jun 19, 2024 at 09:08:14AM +0200, Morten Brørup wrote: > > From: Stephen Hemminger [mailto:step...@networkplumber.org] > 1/5] config/riscv: add flag for using Zbc extension > > > > On Tue, 18 Jun 2024 18:41:29 +0100 > > Daniel Gregory <daniel.greg...@bytedance.com> wrote: > > > > > diff --git a/config/riscv/meson.build b/config/riscv/meson.build > > > index 07d7d9da23..4bda4089bd 100644 > > > --- a/config/riscv/meson.build > > > +++ b/config/riscv/meson.build > > > @@ -26,6 +26,13 @@ flags_common = [ > > > # read from /proc/device-tree/cpus/timebase-frequency. This property > > > is > > > # guaranteed on Linux, as riscv time_init() requires it. > > > ['RTE_RISCV_TIME_FREQ', 0], > > > + > > > + # Use RISC-V Carry-less multiplication extension (Zbc) for hardware > > > + # implementations of CRC-32C (lib/hash/rte_crc_riscv64.h), CRC-32 and > > CRC-16 > > > + # (lib/net/net_crc_zbc.c). Requires intrinsics available in GCC > > > 14.1.0+ > > and > > > + # Clang 18.1.0+ > > > + # Make sure to add '_zbc' to your target's -march below > > > + ['RTE_RISCV_ZBC', false], > > > ] > > > > Please do not add more config options via compile flags. > > It makes it impossible for distros to ship one version. > > > > Instead, detect at compile or runtime > > Build time detection is not possible for cross builds. >
How about build time detection based on the target's configured instruction set (either specified by cross-file or passed in through -Dinstruction_set)? We could have a map from extensions present in the ISA string to compile flags that should be enabled. I suggested this whilst discussing a previous patch adding support for the Zawrs extension, but haven't heard back from Stanisław yet: https://lore.kernel.org/dpdk-dev/20240520094854.GA3672529@ste-uk-lab-gw/ As for runtime detection, newer kernel versions have a hardware probing interface for detecting the presence of extensions, support could be added to rte_cpuflags.c? https://docs.kernel.org/arch/riscv/hwprobe.html In combination, distros on newer kernels could ship a version that has these optimisations baked in that falls back to a generic implementation when the extension is detected to not be present, and systems without the latest GCC/Clang can still compile by specifying a target ISA that doesn't include "_zbc".
