[PATCH v2 5/7] lib: sbi: Workaround for FENCE(.I) errata on C906, C910.

[Benedikt Freisen]

According to the RISCVuzz paper by Thomas et al., the T-Head/XuanTie C906
and C910 cores fail to ignore reserved fields in the "fence" and "fence.i"
encodings and trigger illegal instruction traps if these fields are non-zero,
so address that in the illegal instruction trap handler.

Signed-off-by: Benedikt Freisen <b.freisen at gmx.net>
---
 include/sbi/riscv_encoding.h |  4 ++++
 lib/sbi/sbi_illegal_insn.c   | 17 +++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/include/sbi/riscv_encoding.h b/include/sbi/riscv_encoding.h
index 2970ba0..d956ebb 100644
--- a/include/sbi/riscv_encoding.h
+++ b/include/sbi/riscv_encoding.h
@@ -944,8 +944,12 @@
 #define INSN_MASK_WFI			0xffffff00
 #define INSN_MATCH_WFI			0x10500000
 
+#define INSN_MASK_FENCE			0x0000707f
+#define INSN_MATCH_FENCE		0x0000000f
 #define INSN_MASK_FENCE_TSO		0xfff0707f
 #define INSN_MATCH_FENCE_TSO		0x8330000f
+#define INSN_MASK_FENCE_I		0x0000707f
+#define INSN_MATCH_FENCE_I		0x0000100f
 
 #define INSN_MASK_VECTOR_UNIT_STRIDE		0xfdf0707f
 #define INSN_MASK_VECTOR_FAULT_ONLY_FIRST	0xfdf0707f
diff --git a/lib/sbi/sbi_illegal_insn.c b/lib/sbi/sbi_illegal_insn.c
index fa82264..ed51f4d 100644
--- a/lib/sbi/sbi_illegal_insn.c
+++ b/lib/sbi/sbi_illegal_insn.c
@@ -42,6 +42,23 @@ static int misc_mem_opcode_insn(ulong insn, struct sbi_trap_regs *regs)
 		return 0;
 	}
 
+#ifdef CONFIG_THEAD_C9XX_ERRATA
+	/* Errata workaround: C906, C910 fail to ignore reserved fields
+	 * in the `fence` and `fence.i` encodings. [Thomas2024RISCVuzz] */
+	if ((insn & INSN_MASK_FENCE) == INSN_MATCH_FENCE) {
+		/* NOTE: Emulation should ideally preserve the `pred` and
+		 * `succ` fields, but that is not easily possible here. */
+		mb();
+		regs->mepc += 4;
+		return 0;
+	}
+	if ((insn & INSN_MASK_FENCE_I) == INSN_MATCH_FENCE_I) {
+		RISCV_FENCE_I;
+		regs->mepc += 4;
+		return 0;
+	}
+#endif
+
 	return truly_illegal_insn(insn, regs);
 }
 
-- 
2.51.1