[PATCH v2] riscv: hwprobe: allow querying available RISCV_HWPROBE_KEY_IMA_EXT_0 bits

[Alexandre Ghiti]

Hi Paris,

On 8/13/25 20:05, offtkp wrote:
> When probing for extensions using RISCV_HWPROBE_KEY_IMA_EXT_0, a missing
> bit in the resulting bitmask means the extension is not available or the
> kernel is not recent enough to support the bit. Currently, there's no
> way to differentiate between the two.
>
> This adds a new riscv_hwprobe key, RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL,
> which returns a bitmask of all the queryable extensions supported by the
> kernel in RISCV_HWPROBE_KEY_IMA_EXT_0. This can allow programs to use a
> fallback extension detection method when the bit they want to query is
> not available in the kernel they are running on.


Admittedly, I'm not convinced this is necessary. As you showed in your 
v1, trying to figure out if an extension is available will likely result 
in a SIGILL, so that will kill your application. Why don't you check the 
kernel config or even the hwprobe.h header? Since basically here, you 
use a syscall to return an information that is visible already from 
userspace.

But maybe I'm missing an interesting usecase, just let me know.

Thanks,

Alex


>
> Signed-off-by: offtkp <parisoplop at gmail.com>
> ---
> Changes in v2:
> - convert spaces to tabs
> - use git format-patch to create diff
>
>   Documentation/arch/riscv/hwprobe.rst  | 5 ++++-
>   arch/riscv/include/asm/hwprobe.h      | 3 ++-
>   arch/riscv/include/uapi/asm/hwprobe.h | 2 ++
>   arch/riscv/kernel/sys_hwprobe.c       | 4 ++++
>   4 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/arch/riscv/hwprobe.rst b/Documentation/arch/riscv/hwprobe.rst
> index 2aa9be272d5d..6d77def0a46e 100644
> --- a/Documentation/arch/riscv/hwprobe.rst
> +++ b/Documentation/arch/riscv/hwprobe.rst
> @@ -360,4 +360,7 @@ The following keys are defined:
>   
>       * :c:macro:`RISCV_HWPROBE_VENDOR_EXT_XSFVFWMACCQQQ`: The Xsfvfwmaccqqq
>           vendor extension is supported in version 1.0 of Matrix Multiply Accumulate
> -	Instruction Extensions Specification.
> \ No newline at end of file
> +	Instruction Extensions Specification.
> +
> +* :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL`: A bitmask containing the extensions
> +  that can be probed using the :c:macro:`RISCV_HWPROBE_KEY_IMA_EXT_0` key.
> \ No newline at end of file
> diff --git a/arch/riscv/include/asm/hwprobe.h b/arch/riscv/include/asm/hwprobe.h
> index 7fe0a379474a..501d49b7a02a 100644
> --- a/arch/riscv/include/asm/hwprobe.h
> +++ b/arch/riscv/include/asm/hwprobe.h
> @@ -8,7 +8,8 @@
>   
>   #include <uapi/asm/hwprobe.h>
>   
> -#define RISCV_HWPROBE_MAX_KEY 13
> +#define RISCV_HWPROBE_MAX_KEY 14
> +#define RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE	((1ULL << 59) - 1)
>   
>   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 aaf6ad970499..a3b92df4dc05 100644
> --- a/arch/riscv/include/uapi/asm/hwprobe.h
> +++ b/arch/riscv/include/uapi/asm/hwprobe.h
> @@ -82,6 +82,7 @@ struct riscv_hwprobe {
>   #define		RISCV_HWPROBE_EXT_ZAAMO		(1ULL << 56)
>   #define		RISCV_HWPROBE_EXT_ZALRSC	(1ULL << 57)
>   #define		RISCV_HWPROBE_EXT_ZABHA		(1ULL << 58)
> +/* Change RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE when adding items. */
>   #define RISCV_HWPROBE_KEY_CPUPERF_0	5
>   #define		RISCV_HWPROBE_MISALIGNED_UNKNOWN	(0 << 0)
>   #define		RISCV_HWPROBE_MISALIGNED_EMULATED	(1 << 0)
> @@ -106,6 +107,7 @@ struct riscv_hwprobe {
>   #define RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0	11
>   #define RISCV_HWPROBE_KEY_ZICBOM_BLOCK_SIZE	12
>   #define RISCV_HWPROBE_KEY_VENDOR_EXT_SIFIVE_0	13
> +#define RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL 14
>   /* 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 0b170e18a2be..40e7fa5f85f3 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -310,6 +310,10 @@ static void hwprobe_one_pair(struct riscv_hwprobe *pair,
>   		hwprobe_isa_vendor_ext_thead_0(pair, cpus);
>   		break;
>   
> +	case RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL:
> +		pair->value = RISCV_HWPROBE_KEY_IMA_EXT_0_AVAIL_VALUE;
> +		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
>
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