MIPS: math-emu: Correct the emulation of microMIPS ADDIUPC instruction

Emulate the microMIPS ADDIUPC instruction directly in `mips_dsemul'.  If
executed in the emulation frame, this instruction produces an incorrect
result, because the value of the PC there is not the same as where the
instruction originated.

Reshape code so as to handle all microMIPS cases together.
Signed-off-by: NMaciej W. Rozycki <macro@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12175/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/uapi/asm/inst.h

@@ -799,6 +799,13 @@ struct mm_x_format {		/* Scaled indexed load format (microMIPS) */
 	;)))))
 };
 
+struct mm_a_format {		/* ADDIUPC format (microMIPS) */
+	__BITFIELD_FIELD(unsigned int opcode : 6,
+	__BITFIELD_FIELD(unsigned int rs : 3,
+	__BITFIELD_FIELD(signed int simmediate : 23,
+	;)))
+};
+
 /*
  * microMIPS instruction formats (16-bit length)
  */
@@ -940,6 +947,7 @@ union mips_instruction {
 	struct mm_i_format mm_i_format;
 	struct mm_m_format mm_m_format;
 	struct mm_x_format mm_x_format;
+	struct mm_a_format mm_a_format;
 	struct mm_b0_format mm_b0_format;
 	struct mm_b1_format mm_b1_format;
 	struct mm16_m_format mm16_m_format ;

arch/mips/math-emu/dsemul.c

@@ -43,10 +43,30 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 	int err;
 
 	/* NOP is easy */
-	if ((get_isa16_mode(regs->cp0_epc) && ((ir >> 16) == MM_NOP16)) ||
-	    (ir == 0))
+	if (ir == 0)
 		return -1;
 
+	/* microMIPS instructions */
+	if (get_isa16_mode(regs->cp0_epc)) {
+		union mips_instruction insn = { .word = ir };
+
+		/* NOP16 aka MOVE16 $0, $0 */
+		if ((ir >> 16) == MM_NOP16)
+			return -1;
+
+		/* ADDIUPC */
+		if (insn.mm_a_format.opcode == mm_addiupc_op) {
+			unsigned int rs;
+			s32 v;
+
+			rs = (((insn.mm_a_format.rs + 0x1e) & 0xf) + 2);
+			v = regs->cp0_epc & ~3;
+			v += insn.mm_a_format.simmediate << 2;
+			regs->regs[rs] = (long)v;
+			return -1;
+		}
+	}
+
 	pr_debug("dsemul %lx %lx\n", regs->cp0_epc, cpc);
 
 	/*