target/riscv: Save counter values during countinhibit update

Currently, if a counter monitoring cycle/instret is stopped via
mcountinhibit we just update the state while the value is saved
during the next read. This is not accurate as the read may happen
many cycles after the counter is stopped. Ideally, the read should
return the value saved when the counter is stopped.

Thus, save the value of the counter during the inhibit update
operation and return that value during the read if corresponding bit
in mcountihibit is set.

Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20240711-smcntrpmf_v7-v8-8-b7c38ae7b263@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

3 files changed, 24 insertions, 16 deletions

diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index f515ad0..093c86b 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -176,7 +176,6 @@ typedef struct PMUCTRState {
     target_ulong mhpmcounter_prev;
     /* Snapshort value of a counter in RV32 */
     target_ulong mhpmcounterh_prev;
-    bool started;
     /* Value beyond UINT32_MAX/UINT64_MAX before overflow interrupt trigger */
     target_ulong irq_overflow_left;
 } PMUCTRState;
diff --git a/target/riscv/csr.c b/target/riscv/csr.c
index 85d3f0a..b7a24f9 100644
--- a/target/riscv/csr.c
+++ b/target/riscv/csr.c
@@ -1131,17 +1131,11 @@ static RISCVException riscv_pmu_read_ctr(CPURISCVState *env, target_ulong *val,
 
     if (get_field(env->mcountinhibit, BIT(ctr_idx))) {
         /*
-         * Counter should not increment if inhibit bit is set. We can't really
-         * stop the icount counting. Just return the counter value written by
-         * the supervisor to indicate that counter was not incremented.
+         * Counter should not increment if inhibit bit is set. Just return the
+         * current counter value.
          */
-        if (!counter->started) {
-            *val = ctr_val;
-            return RISCV_EXCP_NONE;
-        } else {
-            /* Mark that the counter has been stopped */
-            counter->started = false;
-        }
+         *val = ctr_val;
+         return RISCV_EXCP_NONE;
     }
 
     /*
@@ -2183,9 +2177,25 @@ static RISCVException write_mcountinhibit(CPURISCVState *env, int csrno,
 
     /* Check if any other counter is also monitoring cycles/instructions */
     for (cidx = 0; cidx < RV_MAX_MHPMCOUNTERS; cidx++) {
-        if (!get_field(env->mcountinhibit, BIT(cidx))) {
             counter = &env->pmu_ctrs[cidx];
-            counter->started = true;
+        if (get_field(env->mcountinhibit, BIT(cidx)) && (val & BIT(cidx))) {
+            /*
+             * Update the counter value for cycle/instret as we can't stop the
+             * host ticks. But we should show the current value at this moment.
+             */
+            if (riscv_pmu_ctr_monitor_cycles(env, cidx) ||
+                riscv_pmu_ctr_monitor_instructions(env, cidx)) {
+                counter->mhpmcounter_val =
+                    riscv_pmu_ctr_get_fixed_counters_val(env, cidx, false) -
+                                           counter->mhpmcounter_prev +
+                                           counter->mhpmcounter_val;
+                if (riscv_cpu_mxl(env) == MXL_RV32) {
+                    counter->mhpmcounterh_val =
+                        riscv_pmu_ctr_get_fixed_counters_val(env, cidx, true) -
+                                                counter->mhpmcounterh_prev +
+                                                counter->mhpmcounterh_val;
+                }
+            }
         }
     }
 
diff --git a/target/riscv/machine.c b/target/riscv/machine.c
index 76f2150..492c2c6 100644
--- a/target/riscv/machine.c
+++ b/target/riscv/machine.c
@@ -320,15 +320,14 @@ static bool pmu_needed(void *opaque)
 
 static const VMStateDescription vmstate_pmu_ctr_state = {
     .name = "cpu/pmu",
-    .version_id = 1,
-    .minimum_version_id = 1,
+    .version_id = 2,
+    .minimum_version_id = 2,
     .needed = pmu_needed,
     .fields = (const VMStateField[]) {
         VMSTATE_UINTTL(mhpmcounter_val, PMUCTRState),
         VMSTATE_UINTTL(mhpmcounterh_val, PMUCTRState),
         VMSTATE_UINTTL(mhpmcounter_prev, PMUCTRState),
         VMSTATE_UINTTL(mhpmcounterh_prev, PMUCTRState),
-        VMSTATE_BOOL(started, PMUCTRState),
         VMSTATE_END_OF_LIST()
     }
 };