Re: [PATCH v3 05/13] riscv: cpufeature: extend riscv_cpufeature_patch_func to all ISA extensions

Heiko Stübner
2023-01-11T23:30:09Z

Hi Jisheng.

Am Mittwoch, 11. Januar 2023, 18:10:19 CET schrieb Jisheng Zhang:
> riscv_cpufeature_patch_func() currently only scans a limited set of
> cpufeatures, explicitly defined with macros. Extend it to probe for all
> ISA extensions.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> Reviewed-by: Heiko Stuebner <heiko@sntech.de>
> ---
> arch/riscv/include/asm/errata_list.h | 9 ++--
> arch/riscv/kernel/cpufeature.c | 63 ++++------------------------
> 2 files changed, 11 insertions(+), 61 deletions(-)

hmmm ... I do see a somewhat big caveat for this.
and would like to take back my Reviewed-by for now

With this change we would limit the patchable cpufeatures to actual
riscv extensions. But cpufeatures can also be soft features like
how performant the core handles unaligned accesses.

See Palmer's series [0].

Also this essentially codifies that each ALTERNATIVE can only ever
be attached to exactly one extension.

But contrary to vendor-errata, it is very likely that we will need
combinations of different extensions for some alternatives in the future.

In my optimization quest, I found that it's actually pretty neat to
convert the errata-id for cpufeatures to a bitfield [1], because then it's
possible to just combine extensions into said bitfield [2]:

ALTERNATIVE_2("nop",
"j strcmp_zbb_unaligned", 0, CPUFEATURE_ZBB | CPUFEATURE_FAST_UNALIGNED, 0, CONFIG_RISCV_ISA_ZBB,
"j variant_zbb", 0, CPUFEATURE_ZBB, CPUFEATURE_FAST_UNALIGNED, CONFIG_RISCV_ISA_ZBB)

[the additional field there models a "not" component]

So I really feel this would limit us quite a bit.

Heiko

[0] https://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git/commit/?h=riscv-hwprobe-v1&id=510c491cb9d87dcbdc91c63558dc704968723240
[1] https://github.com/mmind/linux-riscv/commit/f57a896122ee7e666692079320fc35829434cf96
[2] https://github.com/mmind/linux-riscv/commit/8cef615dab0c00ad68af2651ee5b93d06be17f27#diff-194cb8a86f9fb9b03683295f21c8f46b456a9f94737f01726ddbcbb9e3aace2cR12