cpu-exec: fix missed CPU kick during interrupt injection

The conditional memory barrier not only looks strange but actually is
wrong.

On s390x, I can reproduce interrupts via cpu_interrupt() not leading to
a proper kick out of emulation every now and then. cpu_interrupt() is
especially used for inter CPU communication via SIGP (esp. external
calls and emergency interrupts).

With this patch, I was not able to reproduce. (esp. no stalls or hangs
in the guest).

My setup is s390x MTTCG with 16 VCPUs on 8 CPU host, running make -j16.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171129191319.11483-1-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 9 deletions

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 9b544d88c8..4452cd9856 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -525,19 +525,13 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
                                         TranslationBlock **last_tb)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);
-    int32_t insns_left;
 
     /* Clear the interrupt flag now since we're processing
      * cpu->interrupt_request and cpu->exit_request.
+     * Ensure zeroing happens before reading cpu->exit_request or
+     * cpu->interrupt_request (see also smp_wmb in cpu_exit())
      */
-    insns_left = atomic_read(&cpu->icount_decr.u32);
-    atomic_set(&cpu->icount_decr.u16.high, 0);
-    if (unlikely(insns_left < 0)) {
-        /* Ensure the zeroing of icount_decr comes before the next read
-         * of cpu->exit_request or cpu->interrupt_request.
-         */
-        smp_mb();
-    }
+    atomic_mb_set(&cpu->icount_decr.u16.high, 0);
 
     if (unlikely(atomic_read(&cpu->interrupt_request))) {
         int interrupt_request;