target/riscv: don't read CSR in riscv_csrrw_do64

As per ISA:

"For CSRRWI, if rd=x0, then the instruction shall not read the CSR and
shall not cause any of the side effects that might occur on a CSR read."

trans_csrrwi() and trans_csrrw() call do_csrw() if rd=x0, do_csrw() calls
riscv_csrrw_do64(), via helper_csrw() passing NULL as *ret_value.

Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20230808090914.17634-1-nikita.shubin@maquefel.me>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/csr.c

@@ -3917,21 +3917,27 @@ static RISCVException riscv_csrrw_do64(CPURISCVState *env, int csrno,
                                        target_ulong write_mask)
 {
     RISCVException ret;
-    target_ulong old_value;
+    target_ulong old_value = 0;
 
     /* execute combined read/write operation if it exists */
     if (csr_ops[csrno].op) {
         return csr_ops[csrno].op(env, csrno, ret_value, new_value, write_mask);
     }
 
-    /* if no accessor exists then return failure */
-    if (!csr_ops[csrno].read) {
-        return RISCV_EXCP_ILLEGAL_INST;
-    }
-    /* read old value */
-    ret = csr_ops[csrno].read(env, csrno, &old_value);
-    if (ret != RISCV_EXCP_NONE) {
-        return ret;
+    /*
+     * ret_value == NULL means that rd=x0 and we're coming from helper_csrw()
+     * and we can't throw side effects caused by CSR reads.
+     */
+    if (ret_value) {
+        /* if no accessor exists then return failure */
+        if (!csr_ops[csrno].read) {
+            return RISCV_EXCP_ILLEGAL_INST;
+        }
+        /* read old value */
+        ret = csr_ops[csrno].read(env, csrno, &old_value);
+        if (ret != RISCV_EXCP_NONE) {
+            return ret;
+        }
     }
 
     /* write value if writable and write mask set, otherwise drop writes */