Re: [PATCH v2] riscv: hwprobe: fix has_fpu() to require D extension only

[Conor Dooley <conor@kernel.org>]

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On Tue, Aug 25, 2026 at 08:20:34AM -0600, Ivy Lopez wrote:
> The kernel never supports F without D, since D depends on F. As such,
> has_fpu() checking either extension with '||' is incorrect: it
> reports FPU support when only F is present, which is not sufficient
> for D-dependent state save/restore, and weakens
> RISCV_HWPROBE_IMA_FD semantics to "F or D" instead of "F and D".
> 
> Fix has_fpu() to check D only, which is equivalent to requiring both
> extensions given the dependency. Revert the explicit F && D check in
> sys_hwprobe.c back to calling has_fpu(), which is now correct.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=221874
> Suggested-by: Conor Dooley <conor.dooley@microchip.com>
> Suggested-by: Andreas Schwab <schwab@suse.de>
> Signed-off-by: Ivy Lopez <skunkolee@gmail.com>
> ---
> Changes in v2:
> - Original v1 approach (explicit F && D check in sys_hwprobe.c) was a
>   no-op since F-without-D is structurally impossible. Per Conor and
>   Andreas, fix has_fpu() itself instead, checking D only. Revert the
>   sys_hwprobe.c change back to calling has_fpu().
> - Link to v1: https://lore.kernel.org/r/20260816003216.22534-1-skunkolee@gmail.com

This v2 looks like a diff on top of your v1, but your v1 didn't get
applied (AFAIK) so this patch doesn't make sense.

>  arch/riscv/include/asm/switch_to.h | 4 ++--
>  arch/riscv/kernel/sys_hwprobe.c    | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index 0e71eb82f920..8186cda88e17 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -60,8 +60,8 @@ static inline void __switch_to_fpu(struct task_struct *prev,
>  
>  static __always_inline bool has_fpu(void)
>  {
> -	return riscv_has_extension_likely(RISCV_ISA_EXT_f) ||
> -		riscv_has_extension_likely(RISCV_ISA_EXT_d);
> +	/* D extension depends on F, so checking D alone is sufficient. */
> +	return riscv_has_extension_likely(RISCV_ISA_EXT_d);
>  }
>  #else
>  static __always_inline bool has_fpu(void) { return false; }

> diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> index 7f91beb82a1c..1659d31fd288 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 (riscv_isa_extension_available(NULL, f) && riscv_isa_extension_available(NULL, d))
> +	if (has_fpu())
>  		pair->value |= RISCV_HWPROBE_IMA_FD;
>  
>  	if (riscv_isa_extension_available(NULL, c))

This second portion should be dropped, since it is a reversion to how
things were before.

Additionally, please don't submit new versions as a reply to old ones.

Thanks,
Conor.

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