Re: [PATCH 16/19] riscv: hwprobe: Add vendor extension probing

[Evan Green]

On Thu, Apr 11, 2024 at 9:12 PM Charlie Jenkins <charlie@xxxxxxxxxxxx> wrote:
>
> Add a new hwprobe key "RISCV_HWPROBE_KEY_VENDOR_EXT_0" which allows
> userspace to probe for the new RISCV_ISA_VENDOR_EXT_XTHEADVECTOR vendor
> extension.
>
> Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> ---
>  arch/riscv/include/asm/hwprobe.h      |  4 +--
>  arch/riscv/include/uapi/asm/hwprobe.h | 10 +++++-
>  arch/riscv/kernel/sys_hwprobe.c       | 59 +++++++++++++++++++++++++++++++++--
>  3 files changed, 68 insertions(+), 5 deletions(-)
>
> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 630507dff5ea..e68496b4f8de 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -1,6 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>  /*
> - * Copyright 2023 Rivos, Inc
> + * Copyright 2023-2024 Rivos, Inc
>   */
>
>  #ifndef _ASM_HWPROBE_H
> @@ -8,7 +8,7 @@
>
>  #include <uapi/asm/hwprobe.h>
>
> -#define RISCV_HWPROBE_MAX_KEY 6
> +#define RISCV_HWPROBE_MAX_KEY 7
>
>  static inline bool riscv_hwprobe_key_is_valid(__s64 key)
>  {
> diff --git a/arch/riscv/include/uapi/asm/hwprobe.h b/arch/riscv/include/uapi/asm/hwprobe.h
> index 9f2a8e3ff204..6614d3adfc75 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -1,6 +1,6 @@
>  /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
>  /*
> - * Copyright 2023 Rivos, Inc
> + * Copyright 2023-2024 Rivos, Inc
>   */
>
>  #ifndef _UAPI_ASM_HWPROBE_H
> @@ -67,6 +67,14 @@ struct riscv_hwprobe {
>  #define                RISCV_HWPROBE_MISALIGNED_UNSUPPORTED    (4 << 0)
>  #define                RISCV_HWPROBE_MISALIGNED_MASK           (7 << 0)
>  #define RISCV_HWPROBE_KEY_ZICBOZ_BLOCK_SIZE    6
> +/*
> + * It is not possible for one CPU to have multiple vendor ids, so each vendor
> + * has its own vendor extension "namespace". The keys for each vendor starts
> + * at zero.
> + */
> +#define RISCV_HWPROBE_KEY_VENDOR_EXT_0 7
> + /* T-Head */
> +#define                RISCV_HWPROBE_VENDOR_EXT_XTHEADVECTOR   (1 << 0)
>  /* Increase RISCV_HWPROBE_MAX_KEY when adding items. */
>
>  /* Flags */
> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index e0a42c851511..365ce7380443 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -69,7 +69,8 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
>         if (riscv_isa_extension_available(NULL, c))
>                 pair->value |= RISCV_HWPROBE_IMA_C;
>
> -       if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
> +       if (has_vector() &&
> +           !__riscv_isa_vendor_extension_available(NULL, RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
>                 pair->value |= RISCV_HWPROBE_IMA_V;
>
>         /*
> @@ -112,7 +113,8 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
>                 EXT_KEY(ZACAS);
>                 EXT_KEY(ZICOND);
>
> -               if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) {
> +               if (has_vector() &&
> +                   !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) {
>                         EXT_KEY(ZVBB);
>                         EXT_KEY(ZVBC);
>                         EXT_KEY(ZVKB);
> @@ -139,6 +141,55 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
>         pair->value &= ~missing;
>  }
>
> +static void hwprobe_isa_vendor_ext0(struct riscv_hwprobe *pair,
> +                                   const struct cpumask *cpus)
> +{
> +       int cpu;
> +       u64 missing = 0;
> +
> +       pair->value = 0;
> +
> +       struct riscv_hwprobe mvendorid = {
> +               .key = RISCV_HWPROBE_KEY_MVENDORID,
> +               .value = 0
> +       };
> +
> +       hwprobe_arch_id(&mvendorid, cpus);
> +
> +       /* Set value to zero if CPUs in the set do not have the same vendor. */
> +       if (mvendorid.value == -1ULL)
> +               return;
> +
> +       /*
> +        * Loop through and record vendor extensions that 1) anyone has, and
> +        * 2) anyone doesn't have.
> +        */
> +       for_each_cpu(cpu, cpus) {
> +               struct riscv_isainfo *isavendorinfo = &hart_isa_vendor[cpu];
> +
> +#define VENDOR_EXT_KEY(ext)                                                            \
> +       do {                                                                            \
> +               if (__riscv_isa_vendor_extension_available(isavendorinfo->isa,          \
> +                                                        RISCV_ISA_VENDOR_EXT_##ext))   \
> +                       pair->value |= RISCV_HWPROBE_VENDOR_EXT_##ext;                  \
> +               else                                                                    \
> +                       missing |= RISCV_HWPROBE_VENDOR_EXT_##ext;                      \
> +       } while (false)
> +
> +       /*
> +        * Only use VENDOR_EXT_KEY() for extensions which can be exposed to userspace,
> +        * regardless of the kernel's configuration, as no other checks, besides
> +        * presence in the hart_vendor_isa bitmap, are made.
> +        */
> +       VENDOR_EXT_KEY(XTHEADVECTOR);
> +
> +#undef VENDOR_EXT_KEY

Hey Charlie,
Thanks for writing this up! At the very least I think the
THEAD-specific stuff should probably end up in its own file, otherwise
it'll get chaotic with vendors clamoring to add stuff right here.
What do you think about this approach:
 * We leave RISCV_HWPROBE_MAX_KEY as the max key for the "generic
world", eg 6-ish
 * We define that any key above 0x8000000000000000 is in the vendor
space, so the meaning of the keys depends first on the mvendorid
value.
 * In the kernel code, each new vendor adds on to a global struct,
which might look something like:
struct hwprobe_vendor_space vendor_space[] = {
        {
                .mvendorid = VENDOR_THEAD,
                .max_hwprobe_key = THEAD_MAX_HWPROBE_KEY, // currently
1 or 0x8000000000000001 with what you've got.
                .hwprobe_fn = thead_hwprobe
        },
        ...
};

 * A hwprobe_thead.c implements thead_hwprobe(), and is called
whenever the generic hwprobe encounters a key >=0x8000000000000000.
 * Generic code for setting up the VDSO can then still call the
vendor-specific hwprobe_fn() repeatedly with an "all CPUs" mask from
the base to max_hwprobe_key and set up the cached tables in userspace.
 * Since the VDSO data has limited space we may have to cap the number
of vendor keys we cache to be lower than max_hwprobe_key. Since the
data itself is not exposed to usermode we can raise this cap later if
needed.


-Evan

> +       }
> +
> +       /* Now turn off reporting features if any CPU is missing it. */
> +       pair->value &= ~missing;
> +}
> +
>  static bool hwprobe_ext0_has(const struct cpumask *cpus, unsigned long ext)
>  {
>         struct riscv_hwprobe pair;
> @@ -216,6 +267,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
>                         pair->value = riscv_cboz_block_size;
>                 break;
>
> +       case RISCV_HWPROBE_KEY_VENDOR_EXT_0:
> +               hwprobe_isa_vendor_ext0(pair, cpus);
> +               break;
> +
>         /*
>          * For forward compatibility, unknown keys don't fail the whole
>          * call, but get their element key set to -1 and value set to 0
>
> --
> 2.44.0
>