accel/tcg: Make TCGCPUOps::cpu_exec_halt return bool for whether to halt

The TCGCPUOps::cpu_exec_halt method is called from cpu_handle_halt()
when the CPU is halted, so that a target CPU emulation can do
anything target-specific it needs to do.  (At the moment we only use
this on i386.)

The current specification of the method doesn't allow the target
specific code to do something different if the CPU is about to come
out of the halt state, because cpu_handle_halt() only determines this
after the method has returned.  (If the method called cpu_has_work()
itself this would introduce a potential race if an interrupt arrived
between the target's method implementation checking and
cpu_handle_halt() repeating the check.)

Change the definition of the method so that it returns a bool to
tell cpu_handle_halt() whether to stay in halt or not.

We will want this for the Arm target, where FEAT_WFxT wants to do
some work only for the case where the CPU is in halt but about to
leave it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240430140035.3889879-2-peter.maydell@linaro.org

accel/tcg/cpu-exec.c

@@ -682,11 +682,14 @@ static inline bool cpu_handle_halt(CPUState *cpu)
 #ifndef CONFIG_USER_ONLY
     if (cpu->halted) {
         const TCGCPUOps *tcg_ops = cpu->cc->tcg_ops;
+        bool leave_halt;
 
         if (tcg_ops->cpu_exec_halt) {
-            tcg_ops->cpu_exec_halt(cpu);
+            leave_halt = tcg_ops->cpu_exec_halt(cpu);
+        } else {
+            leave_halt = cpu_has_work(cpu);
         }
-        if (!cpu_has_work(cpu)) {
+        if (!leave_halt) {
             return true;
         }
 

include/hw/core/tcg-cpu-ops.h

@@ -115,8 +115,19 @@ struct TCGCPUOps {
     void (*do_interrupt)(CPUState *cpu);
     /** @cpu_exec_interrupt: Callback for processing interrupts in cpu_exec */
     bool (*cpu_exec_interrupt)(CPUState *cpu, int interrupt_request);
-    /** @cpu_exec_halt: Callback for handling halt in cpu_exec */
-    void (*cpu_exec_halt)(CPUState *cpu);
+    /**
+     * @cpu_exec_halt: Callback for handling halt in cpu_exec.
+     *
+     * The target CPU should do any special processing here that it needs
+     * to do when the CPU is in the halted state.
+     *
+     * Return true to indicate that the CPU should now leave halt, false
+     * if it should remain in the halted state.
+     *
+     * If this method is not provided, the default is to do nothing, and
+     * to leave halt if cpu_has_work() returns true.
+     */
+    bool (*cpu_exec_halt)(CPUState *cpu);
     /**
      * @tlb_fill: Handle a softmmu tlb miss
      *

target/i386/tcg/helper-tcg.h

@@ -39,7 +39,7 @@ QEMU_BUILD_BUG_ON(TCG_PHYS_ADDR_BITS > TARGET_PHYS_ADDR_SPACE_BITS);
  */
 void x86_cpu_do_interrupt(CPUState *cpu);
 #ifndef CONFIG_USER_ONLY
-void x86_cpu_exec_halt(CPUState *cpu);
+bool x86_cpu_exec_halt(CPUState *cpu);
 bool x86_need_replay_interrupt(int interrupt_request);
 bool x86_cpu_exec_interrupt(CPUState *cpu, int int_req);
 #endif

target/i386/tcg/sysemu/seg_helper.c

@@ -128,7 +128,7 @@ void x86_cpu_do_interrupt(CPUState *cs)
     }
 }
 
-void x86_cpu_exec_halt(CPUState *cpu)
+bool x86_cpu_exec_halt(CPUState *cpu)
 {
     if (cpu->interrupt_request & CPU_INTERRUPT_POLL) {
         X86CPU *x86_cpu = X86_CPU(cpu);
@@ -138,6 +138,7 @@ void x86_cpu_exec_halt(CPUState *cpu)
         cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
         bql_unlock();
     }
+    return cpu_has_work(cpu);
 }
 
 bool x86_need_replay_interrupt(int interrupt_request)