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

From: Atish Kumar Patra
Date: Mon Feb 14 2022 - 18:22:36 EST

On Mon, Feb 14, 2022 at 2:22 PM Heiko Stübner <heiko@xxxxxxxxx> wrote:
>
> Am Montag, 14. Februar 2022, 21:42:32 CET schrieb Atish Patra:
> > On Mon, Feb 14, 2022 at 12:24 PM Heiko Stübner <heiko@xxxxxxxxx> wrote:
> > >
> > > Am Montag, 14. Februar 2022, 21:14:13 CET schrieb Atish Patra:
> > > > On Mon, Feb 14, 2022 at 12:06 PM Heiko Stübner <heiko@xxxxxxxxx> 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@xxxxxxxxxxxx>
> > > > >
> > > > > Tested-by: Heiko Stuebner <heiko@xxxxxxxxx>
> > > > >
> > > > >
> > > > > 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?
> > >
> >
> > I think so. I had the same question for the SSTC extension as well.
> > This is what I currently do:
> >
> > 1. Detect menvcfg first, detect stimecmp
> > 2. Enable SSTC feature only if both are available
> > 3. Set the STCE bit in menvcfg if SSTC is available
> >
> > Here is the patch
> > https://github.com/atishp04/opensbi/commit/e6b185821e8302bffdceb4633b413252e0de4889
>
> Hmm, the CBO fields are defined as WARL (write any, read legal),
> so I guess some sort of trap won't work here.
>

Correct. Traps for extensions that introduce new CSRs.
I was suggesting setting the corresponding bits in MENVCFG and reading
it again to check if it sticks.

> The priv-spec only points to the cmo-spec for these bits and the cmo-spec
> does not specifiy what the value should be when cmo is not present.
>
>
> > > > 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 :-)
> > >
> >
> > PBMTE bit in MENVCFG says if PBMTE bit is available or not. OpenSBI
> > needs other way to
> > detect if PBMTE is available.
> >
> > That's why, I think MENVCFG should be set correctly by the hardware
> > upon reset. What do you think
> > about that ? I couldn't find anything related to the reset state for menvcfg.
>
> me neither. Both the priv-spec as well as the cmobase spec do not
> specifiy any reset-values it seems.
>
I have raised an issue in the ISA spec.
https://github.com/riscv/riscv-isa-manual/issues/820

> So I guess in the Qemu case, Qemu needs to set that bit when
> its svpbmt extension is enabled?
>

We can do that if the priv spec is modified to allow that.

>
> 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
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