Disabling ext_zihpm does nothing at this moment. Add support to disable
the hpmcounter3-hpmcounter31 counters if the user disables zihpm.

There is already code in place in target/riscv/csr.c in all predicates
for these counters (ctr() and mctr()) that disables them if
cpu->cfg.pmu_num is zero. Thus, setting cpu->cfg.pmu_num to zero if
'zihpm=false' is enough to disable the extension.

Set cpu->pmu_avail_ctrs mask to zero as well since this is also checked
to verify if the counters exist.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
---
 target/riscv/tcg/tcg-cpu.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/target/riscv/tcg/tcg-cpu.c b/target/riscv/tcg/tcg-cpu.c
index a01b876621..7a4400e2ba 100644
--- a/target/riscv/tcg/tcg-cpu.c
+++ b/target/riscv/tcg/tcg-cpu.c
@@ -549,6 +549,19 @@ void riscv_cpu_validate_set_extensions(RISCVCPU *cpu, Error **errp)
         cpu->cfg.ext_zicntr = false;
     }
 
+    if (cpu->cfg.ext_zihpm && !cpu->cfg.ext_zicsr) {
+        if (cpu_cfg_ext_is_user_set(CPU_CFG_OFFSET(ext_zihpm))) {
+            error_setg(errp, "zihpm requires zicsr");
+            return;
+        }
+        cpu->cfg.ext_zihpm = false;
+    }
+
+    if (!cpu->cfg.ext_zihpm) {
+        cpu->cfg.pmu_num = 0;
+        cpu->pmu_avail_ctrs = 0;
+    }
+
     /*
      * Disable isa extensions based on priv spec after we
      * validated and set everything we need.
-- 
2.41.0