target/riscv: pmp: Ignore writes when RW=01 and MML=0

This patch changes behavior on writing RW=01 to pmpcfg with MML=0.
RWX filed is form of collective WARL with the combination of
pmpcfg.RW=01 remains reserved for future standard use.

According to definition of WARL writing the CSR has no other side
effect. But current implementation change architectural state and
change system behavior. After writing we will get unreadable-unwriteble
region regardless on the previous state.

On the other side WARL said that we should read legal value and nothing
says about what we should write. Current behavior change system state
regardless of whether we read this register or not.

Fixes: ac66f2f0 ("target/riscv: pmp: Ignore writes when RW=01")

Signed-off-by: Ivan Klokov <ivan.klokov@syntacore.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231220153205.11072-1-ivan.klokov@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/pmp.c

@@ -126,7 +126,7 @@ static bool pmp_write_cfg(CPURISCVState *env, uint32_t pmp_index, uint8_t val)
             /* If !mseccfg.MML then ignore writes with encoding RW=01 */
             if ((val & PMP_WRITE) && !(val & PMP_READ) &&
                 !MSECCFG_MML_ISSET(env)) {
-                val &= ~(PMP_WRITE | PMP_READ);
+                return false;
             }
             env->pmp_state.pmp[pmp_index].cfg_reg = val;
             pmp_update_rule_addr(env, pmp_index);