tests/tcg/riscv64: Add test for MEPC bit masking

Add a regression test to verify that MEPC properly masks the lower
bits when an address with mode bits is written to it, as required by
the RISC-V Privileged Architecture specification.

The test sets STVEC to an address with bit 0 set (vectored mode),
triggers an illegal instruction exception, copies STVEC to MEPC in the
trap handler, and verifies that MEPC masks bits [1:0] correctly for
IALIGN=32.

Without the fix, MEPC retains the mode bits (returns non-zero/FAIL).
With the fix, MEPC clears bits [1:0] (returns 0/PASS).

Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20250703182157.281320-3-charmitro@posteo.net>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

tests/tcg/riscv64/Makefile.softmmu-target

@@ -20,5 +20,9 @@ EXTRA_RUNS += run-issue1060
 run-issue1060: issue1060
 	$(call run-test, $<, $(QEMU) $(QEMU_OPTS)$<)
 
+EXTRA_RUNS += run-test-mepc-masking
+run-test-mepc-masking: test-mepc-masking
+	$(call run-test, $<, $(QEMU) $(QEMU_OPTS)$<)
+
 # We don't currently support the multiarch system tests
 undefine MULTIARCH_TESTS

tests/tcg/riscv64/test-mepc-masking.S

@@ -0,0 +1,73 @@
+/*
+ * Test for MEPC masking bug fix
+ *
+ * This test verifies that MEPC properly masks the lower bits according
+ * to the RISC-V specification when vectored mode bits from STVEC are
+ * written to MEPC.
+ */
+
+	.option	norvc
+
+	.text
+	.global _start
+_start:
+	/* Set up machine trap vector */
+	lla	t0, machine_trap_handler
+	csrw	mtvec, t0
+
+	/* Set STVEC with vectored mode (mode bits = 01) */
+	li	t0, 0x80004001
+	csrw	stvec, t0
+
+	/* Clear medeleg to handle exceptions in M-mode */
+	csrw	medeleg, zero
+
+	/* Trigger illegal instruction exception */
+	.word	0xffffffff
+
+test_completed:
+	/* Exit with result in a0 */
+	/* a0 = 0: success (bits [1:0] were masked) */
+	/* a0 != 0: failure (some bits were not masked) */
+	j	_exit
+
+machine_trap_handler:
+	/* Check if illegal instruction (mcause = 2) */
+	csrr	t0, mcause
+	li	t1, 2
+	bne	t0, t1, skip_test
+
+	/* Test: Copy STVEC (with mode bits) to MEPC */
+	csrr	t0, stvec	/* t0 = 0x80004001 */
+	csrw	mepc, t0	/* Write to MEPC */
+	csrr	t1, mepc	/* Read back MEPC */
+
+	/* Check if bits [1:0] are masked (IALIGN=32 without RVC) */
+	andi	a0, t1, 3	/* a0 = 0 if both bits masked correctly */
+
+	/* Set correct return address */
+	lla	t0, test_completed
+	csrw	mepc, t0
+
+skip_test:
+	mret
+
+/* Exit with semihosting */
+_exit:
+	lla	a1, semiargs
+	li	t0, 0x20026	/* ADP_Stopped_ApplicationExit */
+	sd	t0, 0(a1)
+	sd	a0, 8(a1)
+	li	a0, 0x20	/* TARGET_SYS_EXIT_EXTENDED */
+
+	/* Semihosting call sequence */
+	.balign	16
+	slli	zero, zero, 0x1f
+	ebreak
+	srai	zero, zero, 0x7
+	j	.
+
+	.data
+	.balign	8
+semiargs:
+	.space	16