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

[Ivy Lopez]

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@xxxxxxxxxxxxx>
Suggested-by: Andreas Schwab <schwab@xxxxxxx>
Signed-off-by: Ivy Lopez <skunkolee@xxxxxxxxx>
---
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@xxxxxxxxx

 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))
-- 
2.55.0