Re: [PATCH v3 2/3] RISC-V: uapi: add HWCAP for Bitmanip/Scalar Crypto

[Conor Dooley <conor.dooley@microchip.com>]

On Thu, Nov 24, 2022 at 10:30:21AM +0100, Samuel Ortiz wrote:
> On Mon, Jun 13, 2022 at 02:46:35AM +0800, Hongren (Zenithal) Zheng wrote:
> > diff --git a/arch/riscv/include/uapi/asm/hwcap.h b/arch/riscv/include/uapi/asm/hwcap.h
> > index 46dc3f5ee99f..bfed3e5c338c 100644
> > --- a/arch/riscv/include/uapi/asm/hwcap.h
> > +++ b/arch/riscv/include/uapi/asm/hwcap.h
> > @@ -22,4 +22,26 @@
> >  #define COMPAT_HWCAP_ISA_D	(1 << ('D' - 'A'))
> >  #define COMPAT_HWCAP_ISA_C	(1 << ('C' - 'A'))
> >  
> > +/*
> > + * HWCAP2 flags - for elf_hwcap2 (in kernel) and AT_HWCAP2
> > + *
> > + * As only 32 bits of elf_hwcap (in kernel) could be used
> > + * and RISC-V has reserved 26 bits of it, other caps like
> > + * bitmanip and crypto can not be placed in AT_HWCAP
> > + */
> 
> Have we agreed that multi-letter ISA extensions would use hwcap to be
> exposed to userspace? With so many potential extensions, we could
> quickly run out of space on AT_HWCAP2 as well.

Palmer whipped up a PoC hwprobe interface (during Plumbers I think) that
Heiko is currently looking into - I think his motivation is misaligned
access performance. There's a branch but I have no idea if it even
compiles... I'm mostly waiting for whatever Heiko comes up with ;)

https://git.kernel.org/pub/scm/linux/kernel/git/palmer/linux.git/log/?h=riscv-hwprobe-v1

This patchset seems to need a rebase anyway per your other reply, but I
guess that the new proposed interface would be preferable?

Thanks,
Conor.