Re: [PATCH v2 4/6] RISC-V: Implement multi-letter ISA extension probing framework

[Atish Patra <atishp@atishpatra.org>]

On Mon, Feb 14, 2022 at 12:06 PM Heiko Stübner <heiko@sntech.de> wrote:
>
> Am Donnerstag, 10. Februar 2022, 22:40:16 CET schrieb Atish Patra:
> > Multi-letter extensions can be probed using exising
> > riscv_isa_extension_available API now. It doesn't support versioning
> > right now as there is no use case for it.
> > Individual extension specific implementation will be added during
> > each extension support.
> >
> > Signed-off-by: Atish Patra <atishp@rivosinc.com>
>
> Tested-by: Heiko Stuebner <heiko@sntech.de>
>
>
> By the way, does a similar parsing exist for opensbi as well?
> Things like svpbmt as well as zicbom have CSR bits controlling how
> these functions should behave (enabling them, etc), so I guess
> opensbi also needs to parse the extensions from the ISA string?
>
>

No. Currently, OpenSBI relies on the CSR read/write & trap method to
identify the extensions [1].

https://github.com/riscv-software-src/opensbi/blob/master/lib/sbi/sbi_hart.c#L404

In the future, zicbom can be detected in the same manner. However,
svpbmt is a bit tricky as it doesn't
define any new CSR. Do you think OpenSBI needs to detect svpbmt for any reason ?

> Heiko
>
> > ---
> >  arch/riscv/include/asm/hwcap.h | 18 ++++++++++++++++++
> >  arch/riscv/kernel/cpufeature.c | 27 ++++++++++++++++++++++++---
> >  2 files changed, 42 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> > index 5ce50468aff1..170bd80da520 100644
> > --- a/arch/riscv/include/asm/hwcap.h
> > +++ b/arch/riscv/include/asm/hwcap.h
> > @@ -34,7 +34,25 @@ extern unsigned long elf_hwcap;
> >  #define RISCV_ISA_EXT_s              ('s' - 'a')
> >  #define RISCV_ISA_EXT_u              ('u' - 'a')
> >
> > +/*
> > + * Increse this to higher value as kernel support more ISA extensions.
> > + */
> >  #define RISCV_ISA_EXT_MAX    64
> > +#define RISCV_ISA_EXT_NAME_LEN_MAX 32
> > +
> > +/* The base ID for multi-letter ISA extensions */
> > +#define RISCV_ISA_EXT_BASE 26
> > +
> > +/*
> > + * This enum represent the logical ID for each multi-letter RISC-V ISA extension.
> > + * The logical ID should start from RISCV_ISA_EXT_BASE and must not exceed
> > + * RISCV_ISA_EXT_MAX. 0-25 range is reserved for single letter
> > + * extensions while all the multi-letter extensions should define the next
> > + * available logical extension id.
> > + */
> > +enum riscv_isa_ext_id {
> > +     RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
> > +};
> >
> >  unsigned long riscv_isa_extension_base(const unsigned long *isa_bitmap);
> >
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index e9e3b0693d16..469b9739faf7 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -83,7 +83,7 @@ void __init riscv_fill_hwcap(void)
> >
> >       for_each_of_cpu_node(node) {
> >               unsigned long this_hwcap = 0;
> > -             unsigned long this_isa = 0;
> > +             uint64_t this_isa = 0;
> >
> >               if (riscv_of_processor_hartid(node) < 0)
> >                       continue;
> > @@ -169,12 +169,22 @@ void __init riscv_fill_hwcap(void)
> >                       if (*isa != '_')
> >                               --isa;
> >
> > +#define SET_ISA_EXT_MAP(name, bit)                                           \
> > +                     do {                                                    \
> > +                             if ((ext_end - ext == sizeof(name) - 1) &&      \
> > +                                  !memcmp(ext, name, sizeof(name) - 1)) {    \
> > +                                     this_isa |= (1UL << bit);               \
> > +                                     pr_info("Found ISA extension %s", name);\
> > +                             }                                               \
> > +                     } while (false)                                         \
> > +
> >                       if (unlikely(ext_err))
> >                               continue;
> >                       if (!ext_long) {
> >                               this_hwcap |= isa2hwcap[(unsigned char)(*ext)];
> >                               this_isa |= (1UL << (*ext - 'a'));
> >                       }
> > +#undef SET_ISA_EXT_MAP
> >               }
> >
> >               /*
> > @@ -187,10 +197,21 @@ void __init riscv_fill_hwcap(void)
> >               else
> >                       elf_hwcap = this_hwcap;
> >
> > -             if (riscv_isa[0])
> > +             if (riscv_isa[0]) {
> > +#if IS_ENABLED(CONFIG_32BIT)
> > +                     riscv_isa[0] &= this_isa & 0xFFFFFFFF;
> > +                     riscv_isa[1] &= this_isa >> 32;
> > +#else
> >                       riscv_isa[0] &= this_isa;
> > -             else
> > +#endif
> > +             } else {
> > +#if IS_ENABLED(CONFIG_32BIT)
> > +                     riscv_isa[0] = this_isa & 0xFFFFFFFF;
> > +                     riscv_isa[1] = this_isa >> 32;
> > +#else
> >                       riscv_isa[0] = this_isa;
> > +#endif
> > +             }
> >       }
> >
> >       /* We don't support systems with F but without D, so mask those out
> >
>
>
>
>


-- 
Regards,
Atish