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

[Jisheng Zhang]

On Thu, Jan 12, 2023 at 10:21:36AM +0100, Andrew Jones wrote:
> On Thu, Jan 12, 2023 at 12:29:57AM +0100, Heiko Stübner wrote:
> > 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@xxxxxxxxxx>
> > > Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
> > > Reviewed-by: Heiko Stuebner <heiko@xxxxxxxxx>
> > > ---
> > >  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.
> 
> I agree that this needs to be addressed and Jisheng also raised this
> yesterday here [*]. It seems we need the concept of cpufeatures, which
> may be extensions or non-extensions.
> 
> [*] https://lore.kernel.org/all/Y77xyNPNqnFQUqAx@xhacker/
> 
> > 
> > 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.
> 
> One possible approach may be to combine extensions/non-extensions at boot
> time into pseudo-cpufeatures. Then, alternatives can continue attaching to
> a single "feature". (I'm not saying that's a better approach than the
> bitmap, I'm just suggesting it as something else to consider.)

When swtiching pgtable_l4_enabled to static key for the first time, I
suggested bitmap for cpufeatures which cover both ISA extensions
and non-extensions-but-some-cpu-related-features [1],
but it was rejected at that time, it seems we need to revisit the idea.

[1] https://lore.kernel.org/linux-riscv/20220508160749.984-1-jszhang@xxxxxxxxxx/

> 
> Thanks,
> drew
> 
> > 
> > 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
> > 
> >