accel: use store_release/load_acquire for cross-thread exit_request

Reads and writes cpu->exit_request do not use a load-acquire/store-release
pair right now, but this means that cpu_exit() may not write cpu->exit_request
after any flags that are read by the vCPU thread.

Probably everything is protected one way or the other by the BQL, because
cpu->exit_request leads to the slow path, where the CPU thread often takes
the BQL (for example, to go to sleep by waiting on the BQL-protected
cpu->halt_cond); but it's not clear, so use load-acquire/store-release
consistently.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

accel/kvm/kvm-all.c

@@ -3029,10 +3029,6 @@ static void kvm_eat_signals(CPUState *cpu)
 
     if (kvm_immediate_exit) {
         qatomic_set(&cpu->kvm_run->immediate_exit, 0);
-        /* Write kvm_run->immediate_exit before the cpu->exit_request
-         * write in kvm_cpu_exec.
-         */
-        smp_wmb();
         return;
     }
 
@@ -3187,7 +3183,8 @@ int kvm_cpu_exec(CPUState *cpu)
         }
 
         kvm_arch_pre_run(cpu, run);
-        if (qatomic_read(&cpu->exit_request)) {
+        /* Corresponding store-release is in cpu_exit. */
+        if (qatomic_load_acquire(&cpu->exit_request)) {
             trace_kvm_interrupt_exit_request();
             /*
              * KVM requires us to reenter the kernel after IO exits to complete
@@ -3197,13 +3194,15 @@ int kvm_cpu_exec(CPUState *cpu)
             kvm_cpu_kick_self();
         }
 
-        /* Read cpu->exit_request before KVM_RUN reads run->immediate_exit.
-         * Matching barrier in kvm_eat_signals.
-         */
-        smp_rmb();
-
         run_ret = kvm_vcpu_ioctl(cpu, KVM_RUN, 0);
 
+        /*
+         * After writing cpu->exit_request, cpu_exit() sends a signal that writes
+         * kvm->run->immediate_exit.  The signal is already happening after the
+         * write to cpu->exit_request so, if KVM read kvm->run->immediate_exit
+         * as true, cpu->exit_request will always read as true.
+         */
+
         attrs = kvm_arch_post_run(cpu, run);
 
 #ifdef KVM_HAVE_MCE_INJECTION

accel/tcg/cpu-exec.c

@@ -851,8 +851,11 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
     }
 #endif /* !CONFIG_USER_ONLY */
 
-    /* Finally, check if we need to exit to the main loop.  */
-    if (unlikely(qatomic_read(&cpu->exit_request)) || icount_exit_request(cpu)) {
+    /*
+     * Finally, check if we need to exit to the main loop.
+     * The corresponding store-release is in cpu_exit.
+     */
+    if (unlikely(qatomic_load_acquire(&cpu->exit_request)) || icount_exit_request(cpu)) {
         qatomic_set(&cpu->exit_request, 0);
         if (cpu->exception_index == -1) {
             cpu->exception_index = EXCP_INTERRUPT;

accel/tcg/tcg-accel-ops-rr.c

@@ -242,10 +242,17 @@ static void *rr_cpu_thread_fn(void *arg)
             cpu = first_cpu;
         }
 
-        while (cpu && cpu_work_list_empty(cpu) && !cpu->exit_request) {
-            /* Store rr_current_cpu before evaluating cpu_can_run().  */
+        while (cpu && cpu_work_list_empty(cpu)) {
+            /*
+             * Store rr_current_cpu before evaluating cpu->exit_request.
+             * Pairs with rr_kick_next_cpu().
+             */
             qatomic_set_mb(&rr_current_cpu, cpu);
 
+            /* Pairs with store-release in cpu_exit.  */
+            if (qatomic_load_acquire(&cpu->exit_request)) {
+                break;
+            }
             current_cpu = cpu;
 
             qemu_clock_enable(QEMU_CLOCK_VIRTUAL,

hw/core/cpu-common.c

@@ -74,7 +74,8 @@ void cpu_reset_interrupt(CPUState *cpu, int mask)
 
 void cpu_exit(CPUState *cpu)
 {
-    qatomic_set(&cpu->exit_request, 1);
+    /* Ensure cpu_exec will see the reason why the exit request was set.  */
+    qatomic_store_release(&cpu->exit_request, true);
     /* Ensure cpu_exec will see the exit request after TCG has exited.  */
     smp_wmb();
     qatomic_set(&cpu->neg.icount_decr.u16.high, -1);

target/i386/nvmm/nvmm-all.c

@@ -743,7 +743,8 @@ nvmm_vcpu_loop(CPUState *cpu)
 
         nvmm_vcpu_pre_run(cpu);
 
-        if (qatomic_read(&cpu->exit_request)) {
+        /* Corresponding store-release is in cpu_exit. */
+        if (qatomic_load_acquire(&cpu->exit_request)) {
 #if NVMM_USER_VERSION >= 2
             nvmm_vcpu_stop(vcpu);
 #else
@@ -751,8 +752,6 @@ nvmm_vcpu_loop(CPUState *cpu)
 #endif
         }
 
-        /* Read exit_request before the kernel reads the immediate exit flag */
-        smp_rmb();
         ret = nvmm_vcpu_run(mach, vcpu);
         if (ret == -1) {
             error_report("NVMM: Failed to exec a virtual processor,"

target/i386/whpx/whpx-all.c

@@ -1714,7 +1714,8 @@ static int whpx_vcpu_run(CPUState *cpu)
         if (exclusive_step_mode == WHPX_STEP_NONE) {
             whpx_vcpu_pre_run(cpu);
 
-            if (qatomic_read(&cpu->exit_request)) {
+            /* Corresponding store-release is in cpu_exit. */
+            if (qatomic_load_acquire(&cpu->exit_request)) {
                 whpx_vcpu_kick(cpu);
             }
         }