riscv: add support for sysctl unaligned_enabled control

This sysctl tuning option allows the user to disable misaligned access
handling globally on the system. This will also be used by misaligned
detection code to temporarily disable misaligned access handling.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20231004151405.521596-6-cleger@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: clementleger

arch/riscv/Kconfig

@@ -638,6 +638,7 @@ config THREAD_SIZE_ORDER
 
 config RISCV_MISALIGNED
 	bool "Support misaligned load/store traps for kernel and userspace"
+	select SYSCTL_ARCH_UNALIGN_ALLOW
 	default y
 	help
 	  Say Y here if you want the kernel to embed support for misaligned

arch/riscv/kernel/traps_misaligned.c

@@ -396,6 +396,9 @@ union reg_data {
 	u64 data_u64;
 };
 
+/* sysctl hooks */
+int unaligned_enabled __read_mostly = 1;	/* Enabled by default */
+
 int handle_misaligned_load(struct pt_regs *regs)
 {
 	union reg_data val;
@@ -406,6 +409,9 @@ int handle_misaligned_load(struct pt_regs *regs)
 
 	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
 
+	if (!unaligned_enabled)
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;
 
@@ -502,6 +508,9 @@ int handle_misaligned_store(struct pt_regs *regs)
 
 	perf_sw_event(PERF_COUNT_SW_ALIGNMENT_FAULTS, 1, regs, addr);
 
+	if (!unaligned_enabled)
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;