[PATCH v2 16/19] arm64: insn: Allow ADD/SUB (immediate) with LSL #12

[Marc Zyngier]

The encoder for ADD/SUB (immediate) can only cope with 12bit
immediates, while there is an encoding for a 12bit immediate shifted
by 12 bits to the left.

Let's fix this small oversight by allowing the LSL_12 bit to be set.

Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
 arch/arm64/kernel/insn.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
index af29fc3e09a9..b8fb2d89b3a6 100644
--- a/arch/arm64/kernel/insn.c
+++ b/arch/arm64/kernel/insn.c
@@ -35,6 +35,7 @@
 
 #define AARCH64_INSN_SF_BIT	BIT(31)
 #define AARCH64_INSN_N_BIT	BIT(22)
+#define AARCH64_INSN_LSL_12	BIT(22)
 
 static int aarch64_insn_encoding_class[] = {
 	AARCH64_INSN_CLS_UNKNOWN,
@@ -903,9 +904,18 @@ u32 aarch64_insn_gen_add_sub_imm(enum aarch64_insn_register dst,
 		return AARCH64_BREAK_FAULT;
 	}
 
+	/* We can't encode more than a 24bit value (12bit + 12bit shift) */
+	if (imm & ~(BIT(24) - 1))
+		goto out;
+
+	/* If we have something in the top 12 bits... */
 	if (imm & ~(SZ_4K - 1)) {
-		pr_err("%s: invalid immediate encoding %d\n", __func__, imm);
-		return AARCH64_BREAK_FAULT;
+		/* ... and in the low 12 bits -> error */
+		if (imm & (SZ_4K - 1))
+			goto out;
+
+		imm >>= 12;
+		insn |= AARCH64_INSN_LSL_12;
 	}
 
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RD, insn, dst);
@@ -913,6 +923,10 @@ u32 aarch64_insn_gen_add_sub_imm(enum aarch64_insn_register dst,
 	insn = aarch64_insn_encode_register(AARCH64_INSN_REGTYPE_RN, insn, src);
 
 	return aarch64_insn_encode_immediate(AARCH64_INSN_IMM_12, insn, imm);
+
+out:
+	pr_err("%s: invalid immediate encoding %d\n", __func__, imm);
+	return AARCH64_BREAK_FAULT;
 }
 
 u32 aarch64_insn_gen_bitfield(enum aarch64_insn_register dst,
-- 
2.14.2