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

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

[Ivy Lopez]

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.

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

[Conor Dooley]

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

[Ivy Lopez]

On Mon, Aug 17, 2026 at 03:14:36PM +0100, Conor Dooley wrote:
> 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.

Ah ok, thanks for the clarification.. I hadn't realized F-without-D is
already structurally impossible here, so this patch doesn't change
actual behavior.

Would you prefer a v2 that fixes has_fpu() instead (checking D only,
or F && D, whichever matches the intended semantics elsewhere), or
would you rather I just drop this patch since RISCV_HWPROBE_IMA_FD already
behaves correctly in practice?

Happy to send whichever you'd find useful!

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