target/riscv: Add the mcountinhibit CSR

1.11 defines mcountinhibit, which has the same numeric CSR value as
mucounteren from 1.09.1 but has different semantics.  This patch enables
the CSR for 1.11-based targets, which is trivial to implement because
the counters in QEMU never tick (legal according to the spec).

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
[Palmer: Fix counter access semantics, change commit message to indicate
the behavior is fully emulated.]
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

target/riscv/cpu_bits.h

@@ -136,6 +136,7 @@
 #define CSR_MCOUNTEREN      0x306
 
 /* Legacy Counter Setup (priv v1.9.1) */
+/* Update to #define CSR_MCOUNTINHIBIT 0x320 for 1.11.0 */
 #define CSR_MUCOUNTEREN     0x320
 #define CSR_MSCOUNTEREN     0x321
 #define CSR_MHCOUNTEREN     0x322

target/riscv/csr.c

@@ -56,6 +56,15 @@ static int fs(CPURISCVState *env, int csrno)
 static int ctr(CPURISCVState *env, int csrno)
 {
 #if !defined(CONFIG_USER_ONLY)
+    /*
+     * The counters are always enabled on newer priv specs, as the CSR has
+     * changed from controlling that the counters can be read to controlling
+     * that the counters increment.
+     */
+    if (env->priv_ver > PRIV_VERSION_1_09_1) {
+        return 0;
+    }
+
     uint32_t ctr_en = ~0u;
 
     if (env->priv < PRV_M) {
@@ -461,18 +470,22 @@ static int write_mcounteren(CPURISCVState *env, int csrno, target_ulong val)
     return 0;
 }
 
+/* This regiser is replaced with CSR_MCOUNTINHIBIT in 1.11.0 */
 static int read_mscounteren(CPURISCVState *env, int csrno, target_ulong *val)
 {
-    if (env->priv_ver > PRIV_VERSION_1_09_1) {
+    if (env->priv_ver > PRIV_VERSION_1_09_1
+        && env->priv_ver < PRIV_VERSION_1_11_0) {
         return -1;
     }
     *val = env->mcounteren;
     return 0;
 }
 
+/* This regiser is replaced with CSR_MCOUNTINHIBIT in 1.11.0 */
 static int write_mscounteren(CPURISCVState *env, int csrno, target_ulong val)
 {
-    if (env->priv_ver > PRIV_VERSION_1_09_1) {
+    if (env->priv_ver > PRIV_VERSION_1_09_1
+        && env->priv_ver < PRIV_VERSION_1_11_0) {
         return -1;
     }
     env->mcounteren = val;