[PATCH] riscv: misaligned: Make enabling delegation depend on BROKEN

[Vivian Wang]

The unaligned access emulation code in Linux has various deficiencies.
For example, it doesn't emulate vector instructions [1], and doesn't
emulate KVM guest accesses. Therefore, requesting misaligned exception
delegation with SBI FWFT actually regresses userspace and KVM guest
behavior. Until Linux can handle it properly, guard these sbi_fwft_set()
calls behind RISCV_SBI_FWFT_DELEGATE_MISALIGNED, which in turn depends
on BROKEN.

The rest of the existing code proceeds as before, except as if
SBI_FWFT_MISALIGNED_EXC_DELEG is not available, to handle any remaining
address misaligned exceptions on a best-effort basis. The KVM SBI FWFT
implementation is also not touched, but it is disabled if the firmware
emulates unaligned accesses.

Cc: stable at vger.kernel.org
Fixes: cf5a8abc6560 ("riscv: misaligned: request misaligned exception from SBI")
Reported-by: Songsong Zhang <U2FsdGVkX1 at gmail.com> # KVM
Link: https://lore.kernel.org/linux-riscv/38ce44c1-08cf-4e3f-8ade-20da224f529c@iscas.ac.cn/ [1]
Signed-off-by: Vivian Wang <wangruikang at iscas.ac.cn>
---
Clément: Sorry to call what you did broken, but it really is breaking
on real hardware out there. I think this is the right way for now.
---
 arch/riscv/Kconfig                   | 14 ++++++++++++++
 arch/riscv/kernel/traps_misaligned.c |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 90c531e6abf5..8ad1f13c170e 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -941,6 +941,20 @@ config RISCV_VECTOR_MISALIGNED
 	help
 	  Enable detecting support for vector misaligned loads and stores.
 
+config RISCV_SBI_FWFT_DELEGATE_MISALIGNED
+	bool "Request firmware delegation of unaligned access exceptions"
+	depends on RISCV_SBI
+	depends on BROKEN
+	help
+	  Use SBI FWFT to request delegation of load address misaligned and
+	  store address misaligned exceptions, if possible, and prefer Linux
+	  kernel emulation of these accesses to firmware emulation.
+
+	  Since the Linux kernel's emulation is incomplete, enabling this may
+	  cause unexpected userspace and KVM guest crashes.
+
+	  If you don't know what to do here, say N.
+
 choice
 	prompt "Unaligned Accesses Support"
 	default RISCV_PROBE_UNALIGNED_ACCESS
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index 2a27d3ff4ac6..81b7682e6c6d 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -584,7 +584,7 @@ static int cpu_online_check_unaligned_access_emulated(unsigned int cpu)
 
 static bool misaligned_traps_delegated;
 
-#ifdef CONFIG_RISCV_SBI
+#if defined(CONFIG_RISCV_SBI_FWFT_DELEGATE_MISALIGNED)
 
 static int cpu_online_sbi_unaligned_setup(unsigned int cpu)
 {

---
base-commit: f338e77383789c0cae23ca3d48adcc5e9e137e3c
change-id: 20260330-riscv-misaligned-dont-delegate-3cf98c76ee08

Best regards,
-- 
Vivian "dramforever" Wang