[RFC PATCH 2/6] riscv: add support for misaligned handling in S-mode

[Clément Léger]

Misalignment handling is only supported for M-mode and uses direct
accesses to user memory. in S-mode, this requires to use the
get_user()/put_user() accessors. Implement load_u8(), store_u8() and
get_insn() using these accessors. Also, use CSR_TVAL instead of
hardcoded mtval in csr_read() call which will work for both S-mode and
M-mode. When used in S-mode, we do not handle misaligned accesses that
are triggered from kernel mode.

Signed-off-by: Clément Léger <cleger at rivosinc.com>
---
 arch/riscv/kernel/Makefile           |   2 +-
 arch/riscv/kernel/traps_misaligned.c | 111 +++++++++++++++++++++++----
 2 files changed, 99 insertions(+), 14 deletions(-)

diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index 153864e4f399..61bad09280a6 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -56,9 +56,9 @@ obj-y	+= stacktrace.o
 obj-y	+= cacheinfo.o
 obj-y	+= patch.o
 obj-y	+= probes/
+obj-y	+= traps_misaligned.o
 obj-$(CONFIG_MMU) += vdso.o vdso/
 
-obj-$(CONFIG_RISCV_M_MODE)	+= traps_misaligned.o
 obj-$(CONFIG_FPU)		+= fpu.o
 obj-$(CONFIG_SMP)		+= smpboot.o
 obj-$(CONFIG_SMP)		+= smp.o
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index e7bfb33089c1..e4a273ab77c9 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -12,6 +12,7 @@
 #include <asm/processor.h>
 #include <asm/ptrace.h>
 #include <asm/csr.h>
+#include <asm/bug.h>
 
 #define INSN_MATCH_LB			0x3
 #define INSN_MASK_LB			0x707f
@@ -151,21 +152,25 @@
 #define PRECISION_S 0
 #define PRECISION_D 1
 
-static inline u8 load_u8(const u8 *addr)
+#ifdef CONFIG_RISCV_M_MODE
+static inline int load_u8(const u8 *addr, u8 *r_val)
 {
 	u8 val;
 
 	asm volatile("lbu %0, %1" : "=&r" (val) : "m" (*addr));
+	*r_val = val;
 
-	return val;
+	return 0;
 }
 
-static inline void store_u8(u8 *addr, u8 val)
+static inline int store_u8(u8 *addr, u8 val)
 {
 	asm volatile ("sb %0, %1\n" : : "r" (val), "m" (*addr));
+
+	return 0;
 }
 
-static inline ulong get_insn(ulong mepc)
+static inline int get_insn(ulong mepc, ulong *r_insn)
 {
 	register ulong __mepc asm ("a2") = mepc;
 	ulong val, rvc_mask = 3, tmp;
@@ -194,9 +199,63 @@ static inline ulong get_insn(ulong mepc)
 	: [addr] "r" (__mepc), [rvc_mask] "r" (rvc_mask),
 	  [xlen_minus_16] "i" (XLEN_MINUS_16));
 
-	return val;
+	*r_insn = val;
+
+	return 0;
+}
+#else
+static inline int load_u8(const u8 *addr, u8 *r_val)
+{
+	return __get_user(*r_val, addr);
+}
+
+static inline int store_u8(u8 *addr, u8 val)
+{
+	return __put_user(val, addr);
 }
 
+static inline int get_insn(ulong mepc, ulong *r_insn)
+{
+	ulong insn = 0;
+
+	if (mepc & 0x2) {
+		ulong tmp = 0;
+		u16 __user *insn_addr = (u16 __user *)mepc;
+
+		if (__get_user(insn, insn_addr))
+			return -EFAULT;
+		/* __get_user() uses regular "lw" which sign extend the loaded
+		 * value make sure to clear higher order bits in case we "or" it
+		 * below with the upper 16 bits half.
+		 */
+		insn &= GENMASK(15, 0);
+		if ((insn & __INSN_LENGTH_MASK) != __INSN_LENGTH_32) {
+			*r_insn = insn;
+			return 0;
+		}
+		insn_addr++;
+		if (__get_user(tmp, insn_addr))
+			return -EFAULT;
+		*r_insn = (tmp << 16) | insn;
+
+		return 0;
+	} else {
+		u32 __user *insn_addr = (u32 __user *)mepc;
+
+		if (__get_user(insn, insn_addr))
+			return -EFAULT;
+		if ((insn & __INSN_LENGTH_MASK) == __INSN_LENGTH_32) {
+			*r_insn = insn;
+			return 0;
+		}
+		insn &= GENMASK(15, 0);
+		*r_insn = insn;
+
+		return 0;
+	}
+}
+#endif
+
 union reg_data {
 	u8 data_bytes[8];
 	ulong data_ulong;
@@ -207,10 +266,21 @@ int handle_misaligned_load(struct pt_regs *regs)
 {
 	union reg_data val;
 	unsigned long epc = regs->epc;
-	unsigned long insn = get_insn(epc);
-	unsigned long addr = csr_read(mtval);
+	unsigned long insn;
+	unsigned long addr;
 	int i, fp = 0, shift = 0, len = 0;
 
+	/*
+	 * When running in supervisor mode, we only handle misaligned accesses
+	 * triggered from user mode.
+	 */
+	if (!IS_ENABLED(CONFIG_RISCV_M_MODE) && !user_mode(regs))
+		return -1;
+
+	if (get_insn(epc, &insn))
+		return -1;
+
+	addr = csr_read(CSR_TVAL);
 	regs->epc = 0;
 
 	if ((insn & INSN_MASK_LW) == INSN_MATCH_LW) {
@@ -274,8 +344,10 @@ int handle_misaligned_load(struct pt_regs *regs)
 	}
 
 	val.data_u64 = 0;
-	for (i = 0; i < len; i++)
-		val.data_bytes[i] = load_u8((void *)(addr + i));
+	for (i = 0; i < len; i++) {
+		if (load_u8((void *)(addr + i), &val.data_bytes[i]))
+			return -1;
+	}
 
 	if (fp)
 		return -1;
@@ -290,10 +362,21 @@ int handle_misaligned_store(struct pt_regs *regs)
 {
 	union reg_data val;
 	unsigned long epc = regs->epc;
-	unsigned long insn = get_insn(epc);
-	unsigned long addr = csr_read(mtval);
+	unsigned long insn;
+	unsigned long addr;
 	int i, len = 0;
 
+	/*
+	 * When running in supervisor mode, we only handle misaligned accesses
+	 * triggered from user mode.
+	 */
+	if (!IS_ENABLED(CONFIG_RISCV_M_MODE) && !user_mode(regs))
+		return -1;
+
+	if (get_insn(epc, &insn))
+		return -1;
+
+	addr = csr_read(CSR_TVAL);
 	regs->epc = 0;
 
 	val.data_ulong = GET_RS2(insn, regs);
@@ -327,8 +410,10 @@ int handle_misaligned_store(struct pt_regs *regs)
 		return -1;
 	}
 
-	for (i = 0; i < len; i++)
-		store_u8((void *)(addr + i), val.data_bytes[i]);
+	for (i = 0; i < len; i++) {
+		if (store_u8((void *)(addr + i), val.data_bytes[i]))
+			return -1;
+	}
 
 	regs->epc = epc + INSN_LEN(insn);
 
-- 
2.40.1