Re: [PATCH] riscv: hwprobe: require both F and D extensions for RISCV_HWPROBE_IMA_FD

[Conor Dooley <conor.dooley@microchip.com>]

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On Sat, Aug 15, 2026 at 06:32:16PM -0600, Ivy Lopez wrote:
> Documentation/arch/riscv/hwprobe.rst states that RISCV_HWPROBE_IMA_FD
> means both the F and D extensions are supported. However, the
> implementation uses has_fpu(), which returns true if either extension
> is present, since that's sufficient to determine whether FPU state
> needs to be saved/restored. Reusing it here silently weakens the
> hwprobe semantics from "F and D" to "F or D", giving userspace no way
> to know which extension is actually present when only one is.

The kernel never supports F without D, so this cannot occur.
riscv_isa_extension_available(NULL, f) always returns false if D is not
also present.

If anything, the code in has_fpu() should be changed to only check D and
not F to match expectations elsewhere, or F and D for the same reason.

> 
> Check both extensions explicitly with riscv_isa_extension_available(),
> matching the pattern already used for the neighboring C and V checks.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=221874
> Signed-off-by: Ivy Lopez <skunkolee@gmail.com>
> ---
>  arch/riscv/kernel/sys_hwprobe.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index 1659d31fd288..7f91beb82a1c 100644
> --- a/arch/riscv/kernel/sys_hwprobe.c
> +++ b/arch/riscv/kernel/sys_hwprobe.c
> @@ -85,7 +85,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
>  	u64 missing = 0;
>  
>  	pair->value = 0;
> -	if (has_fpu())
> +	if (riscv_isa_extension_available(NULL, f) && riscv_isa_extension_available(NULL, d))
>  		pair->value |= RISCV_HWPROBE_IMA_FD;
>  
>  	if (riscv_isa_extension_available(NULL, c))
> -- 
> 2.55.0
> 

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