diff mbox series

[v3,09/21] target/arm: Access halt state atomically

Message ID 20260923171116.31276-10-philmd@oss.qualcomm.com
State New
Headers show
Series accel/tcg: Make halt-to-exec transition explicit and remove cpu_exec_halt | expand

Commit Message

Philippe Mathieu-Daudé Sept. 23, 2026, 5:11 p.m. UTC 
arm_cpu_has_work() runs without the BQL held and can inspect
halt_reason and event_register while other CPU contexts update
them. The WFxT timer may also consume HALT_WFE asynchronously.

Use atomic accesses for the halt state, including the WFI/WFE
and halt-exit stores. This keeps the halt/wakeup protocol
race-free and matches the atomic state transitions used by the
asynchronous wake-up paths.

Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
---
v3: Dropped rth's R-b because of HELPER()s call also updated
---
 target/arm/cpu.c           | 7 ++++---
 target/arm/tcg/op_helper.c | 8 ++++----
 2 files changed, 8 insertions(+), 7 deletions(-)

Comments

Richard Henderson Sept. 23, 2026, 5:29 p.m. UTC | #1 
On 9/23/26 10:11, Philippe Mathieu-Daudé wrote:
> arm_cpu_has_work() runs without the BQL held and can inspect
> halt_reason and event_register while other CPU contexts update
> them. The WFxT timer may also consume HALT_WFE asynchronously.
> 
> Use atomic accesses for the halt state, including the WFI/WFE
> and halt-exit stores. This keeps the halt/wakeup protocol
> race-free and matches the atomic state transitions used by the
> asynchronous wake-up paths.
> 
> Signed-off-by: Philippe Mathieu-Daudé<philmd@oss.qualcomm.com>
> ---
> v3: Dropped rth's R-b because of HELPER()s call also updated
> ---
>   target/arm/cpu.c           | 7 ++++---
>   target/arm/tcg/op_helper.c | 8 ++++----
>   2 files changed, 8 insertions(+), 7 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~
diff mbox series

Patch

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 5cfd3bcfc8e..090470da40a 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -144,20 +144,21 @@  int arm_cpu_mmu_index(CPUState *cs, bool ifetch)
 static bool arm_cpu_has_work(CPUState *cs)
 {
     ARMCPU *cpu = ARM_CPU(cs);
+    ARMHaltReason halt_reason = qatomic_read(&cpu->env.halt_reason);
 
     /*
      * Only another PSCI call can wake the CPU up in which case the
      * power_state would be set by arm_set_cpu_on_and_reset_async_work()
      */
     if (qatomic_read(&cpu->power_state) == PSCI_OFF) {
-        g_assert(cpu->env.halt_reason == HALT_PSCI);
+        g_assert(halt_reason == HALT_PSCI);
         return false;
     }
 
     /*
      * A wake-up event should only wake us if we are halted on a WFE
      */
-    if (cpu->env.halt_reason == HALT_WFE && cpu->env.event_register) {
+    if (halt_reason == HALT_WFE && qatomic_read(&cpu->env.event_register)) {
         return true;
     }
 
@@ -882,7 +883,7 @@  bool arm_cpu_exec_halt(CPUState *cs)
             timer_del(cpu->wfxt_timer);
         }
         /* clear the halt reason */
-        cpu->env.halt_reason = NOT_HALTED;
+        qatomic_set(&cpu->env.halt_reason, NOT_HALTED);
     }
     return leave_halt;
 }
diff --git a/target/arm/tcg/op_helper.c b/target/arm/tcg/op_helper.c
index c2b09176cb1..643b1482523 100644
--- a/target/arm/tcg/op_helper.c
+++ b/target/arm/tcg/op_helper.c
@@ -398,7 +398,7 @@  void HELPER(wfi)(CPUARMState *env, uint32_t insn_len)
                         target_el);
     }
 
-    env->halt_reason = HALT_WFI;
+    qatomic_set(&env->halt_reason, HALT_WFI);
     cs->exception_index = EXCP_HLT;
     cs->halted = 1;
     cpu_loop_exit(cs);
@@ -460,7 +460,7 @@  void HELPER(wfit)(CPUARMState *env, uint32_t rd)
     } else {
         timer_mod(cpu->wfxt_timer, nexttick);
     }
-    env->halt_reason = HALT_WFI;
+    qatomic_set(&env->halt_reason, HALT_WFI);
     cs->exception_index = EXCP_HLT;
     cs->halted = 1;
     cpu_loop_exit(cs);
@@ -629,7 +629,7 @@  void HELPER(wfe)(CPUARMState *env, uint32_t insn_len)
         }
     }
 
-    env->halt_reason = HALT_WFE;
+    qatomic_set(&env->halt_reason, HALT_WFE);
     cs->exception_index = EXCP_HLT;
     cs->halted = 1;
     cpu_loop_exit(cs);
@@ -723,7 +723,7 @@  void HELPER(wfet)(CPUARMState *env, uint32_t rd)
         }
     }
 
-    env->halt_reason = HALT_WFE;
+    qatomic_set(&env->halt_reason, HALT_WFE);
     cs->exception_index = EXCP_HLT;
     cs->halted = 1;
     cpu_loop_exit(cs);