[PATCH v2 1/2] riscv: csr: do not drop C bit on misa write

[Vladimir Isaev <vvisaev@gmail.com>]

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