* [PATCH v2 1/2] riscv: csr: do not drop C bit on misa write
2026-08-17 15:06 [PATCH v2 0/2] Fix alignment check for misa write Vladimir Isaev
@ 2026-08-17 15:06 ` Vladimir Isaev
2026-08-17 15:06 ` [PATCH v2 2/2] tests/tcg/riscv64: add misa write test Vladimir Isaev
1 sibling, 0 replies; 3+ messages in thread
From: Vladimir Isaev @ 2026-08-17 15:06 UTC (permalink / raw)
To: qemu-devel
Cc: palmer, alistair.francis, liwei1518, daniel.barboza, zhiwei_liu,
chao.liu, qemu-riscv, joel, npiggin, Vladimir Isaev
According to spec:
> Writing misa may increase IALIGN, e.g., by disabling the "C" extension.
> If an instruction that would write misa increases IALIGN, and the
> subsequent instruction’s address is not IALIGN-bit aligned, the
> write to misa is suppressed, leaving misa unchanged.
So attempt to disable C extension if next instruction is not aligned should not
change the misa.
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Vladimir Isaev <vvisaev@gmail.com>
---
target/riscv/tcg/csr.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/target/riscv/tcg/csr.c b/target/riscv/tcg/csr.c
index 36f2004bc5..dbfb3381ef 100644
--- a/target/riscv/tcg/csr.c
+++ b/target/riscv/tcg/csr.c
@@ -2182,9 +2182,13 @@ static RISCVException write_misa(CPURISCVState *env, int csrno,
/* Mask extensions that are not supported by this hart */
val &= env->misa_ext_mask;
- /* Suppress 'C' if next instruction is not aligned. */
- if ((val & RVC) && (get_next_pc(env, ra) & 3) != 0) {
- val &= ~RVC;
+ /* drop write if RVC is cleared and next instruction is not aligned */
+ if ((env->misa_ext & RVC) && !(val & RVC) &&
+ (get_next_pc(env, ra) & 3) != 0) {
+ qemu_log_mask(LOG_GUEST_ERROR, "Unable to write MISA ext value "
+ "0x%x, MISA.C disable failed\n", env->misa_ext);
+
+ return RISCV_EXCP_NONE;
}
/* Disable RVG if any of its dependencies are disabled */
--
2.55.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* [PATCH v2 2/2] tests/tcg/riscv64: add misa write test
2026-08-17 15:06 [PATCH v2 0/2] Fix alignment check for misa write Vladimir Isaev
2026-08-17 15:06 ` [PATCH v2 1/2] riscv: csr: do not drop C bit on " Vladimir Isaev
@ 2026-08-17 15:06 ` Vladimir Isaev
1 sibling, 0 replies; 3+ messages in thread
From: Vladimir Isaev @ 2026-08-17 15:06 UTC (permalink / raw)
To: qemu-devel
Cc: palmer, alistair.francis, liwei1518, daniel.barboza, zhiwei_liu,
chao.liu, qemu-riscv, joel, npiggin, Vladimir Isaev
Link: https://lore.kernel.org/r/20260321144554.606417-2-npiggin@gmail.com
Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Vladimir Isaev <vvisaev@gmail.com>
---
tests/tcg/riscv64/Makefile.softmmu-target | 4 ++
tests/tcg/riscv64/test-misa-w.S | 88 +++++++++++++++++++++++
2 files changed, 92 insertions(+)
create mode 100644 tests/tcg/riscv64/test-misa-w.S
diff --git a/tests/tcg/riscv64/Makefile.softmmu-target b/tests/tcg/riscv64/Makefile.softmmu-target
index 82be8a2c91..2c7f00976c 100644
--- a/tests/tcg/riscv64/Makefile.softmmu-target
+++ b/tests/tcg/riscv64/Makefile.softmmu-target
@@ -41,5 +41,9 @@ comma:= ,
run-test-crc32: test-crc32
$(call run-test, $<, $(QEMU) -cpu rv64$(comma)xlrbr=true $(QEMU_OPTS)$<)
+EXTRA_RUNS += run-test-misa-w
+run-test-misa-w: test-misa-w
+ $(call run-test, $<, $(QEMU) -cpu rv64$(comma)x-misa-w=true$(comma)c=true$(comma)v=true $(QEMU_OPTS)$<)
+
# We don't currently support the multiarch system tests
undefine MULTIARCH_TESTS
diff --git a/tests/tcg/riscv64/test-misa-w.S b/tests/tcg/riscv64/test-misa-w.S
new file mode 100644
index 0000000000..7f1eb30023
--- /dev/null
+++ b/tests/tcg/riscv64/test-misa-w.S
@@ -0,0 +1,88 @@
+/*
+ * Test for MISA changing C and related IALIGN alignment cases
+ *
+ * This test verifies that the "C" extension can be cleared and set in MISA,
+ * that a branch to 2-byte aligned instructions can be executed when "C" is
+ * enabled, and that a write to MISA which would increase IALIGN and cause
+ * the next instruction to be unaligned is ignored.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#define RVC (1 << ('C'-'A'))
+#define RVV (1 << ('V'-'A'))
+
+.option norvc
+ .text
+ .global _start
+_start:
+ lla t0, trap
+ csrw mtvec, t0
+
+ csrr t0, misa
+ li t1, RVC
+ not t1, t1
+ and t0, t0, t1
+ csrw misa, t0
+ csrr t1, misa
+ li a0, 2 # fail code
+ bne t0, t1, _exit # Could not clear RVC in MISA
+
+ li t1, RVC
+ or t0, t0, t1
+ csrw misa, t0
+ csrr t1, misa
+ li a0, 3 # fail code
+ bne t0, t1, _exit # Could not set RVC in MISA
+
+ j unalign
+. = . + 2
+unalign:
+
+ li t1, RVC
+ not t1, t1
+ and t0, t0, t1
+ csrw misa, t0
+ csrr t1, misa
+ li a0, 4 # fail code
+ beq t0, t1, _exit # Was able to clear RVC in MISA
+
+ li t0, (RVC|RVV)
+ not t0, t0
+ and t0, t0, t1
+ csrw misa, t0
+ csrr t0, misa
+ li a0, 5 # fail code
+ bne t0, t1, _exit # MISA write was not ignored (RVV was cleared)
+
+ j realign
+. = . + 2
+realign:
+
+ # Success!
+ li a0, 0
+ j _exit
+
+trap:
+ # Any trap is a fail code 1
+ li a0, 1
+
+# Exit code in a0
+_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 16
+semiargs:
+ .space 16
--
2.55.0
^ permalink raw reply related [flat|nested] 3+ messages in thread