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

["Heiko Stübner" <heiko@sntech.de>]

Am Montag, 14. Februar 2022, 21:14:13 CET schrieb Atish Patra:
> 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

I guess my question is more, who is supposed to set CBIE, CBCFE bits in the
ENVCFG CSR. I.e. at it's default settings CMO instructions will cause
illegal instructions until the level above does allow them.

When the kernel wants to call a cache-invalidate, from my reading menvcfg
needs to be modified accordingly - which would fall in SBI's court?


> 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 ?

There is the PBMTE bit in MENVCFG, which I found while looking through the
zicbom-parts, which is supposed to "control wheter svpbmt is available for
use". So I guess the question is the same as above :-)


Heiko


> > 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
> > >
> >
> >
> >
> >
> 
> 
>