target/riscv: Add support for disabling/enabling Counters

Add support for disabling/enabling the "Counters" extension.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

target/riscv/cpu.c

@@ -440,6 +440,7 @@ static Property riscv_cpu_properties[] = {
     DEFINE_PROP_BOOL("c", RISCVCPU, cfg.ext_c, true),
     DEFINE_PROP_BOOL("s", RISCVCPU, cfg.ext_s, true),
     DEFINE_PROP_BOOL("u", RISCVCPU, cfg.ext_u, true),
+    DEFINE_PROP_BOOL("Counters", RISCVCPU, cfg.ext_counters, true),
     DEFINE_PROP_STRING("priv_spec", RISCVCPU, cfg.priv_spec),
     DEFINE_PROP_BOOL("mmu", RISCVCPU, cfg.mmu, true),
     DEFINE_PROP_BOOL("pmp", RISCVCPU, cfg.pmp, true),

target/riscv/cpu.h

@@ -222,6 +222,7 @@ typedef struct RISCVCPU {
         bool ext_c;
         bool ext_s;
         bool ext_u;
+        bool ext_counters;
 
         char *priv_spec;
         char *user_spec;

target/riscv/csr.c

@@ -56,17 +56,24 @@ static int fs(CPURISCVState *env, int csrno)
 static int ctr(CPURISCVState *env, int csrno)
 {
 #if !defined(CONFIG_USER_ONLY)
+    CPUState *cs = env_cpu(env);
+    RISCVCPU *cpu = RISCV_CPU(cs);
+    uint32_t ctr_en = ~0u;
+
+    if (!cpu->cfg.ext_counters) {
+        /* The Counters extensions is not enabled */
+        return -1;
+    }
+
     /*
-     * 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.
+     * The counters are always enabled at run time 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) {
         ctr_en &= env->mcounteren;
     }