kvm: Flush coalesced MMIO buffer periodly

The default action of coalesced MMIO is, cache the writing in buffer, until:
1. The buffer is full.
2. Or the exit to QEmu due to other reasons.

But this would result in a very late writing in some condition.
1. The each time write to MMIO content is small.
2. The writing interval is big.
3. No need for input or accessing other devices frequently.

This issue was observed in a experimental embbed system. The test image
simply print "test" every 1 seconds. The output in QEmu meets expectation,
but the output in KVM is delayed for seconds.

Per Avi's suggestion, I hooked flushing coalesced MMIO buffer in VGA update
handler. By this way, We don't need vcpu explicit exit to QEmu to
handle this issue.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 15 insertions, 8 deletions

diff --git a/kvm-all.c b/kvm-all.c
index 15ec38e631..f8350c9580 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -59,6 +59,9 @@ struct KVMState
     int vmfd;
     int regs_modified;
     int coalesced_mmio;
+#ifdef KVM_CAP_COALESCED_MMIO
+    struct kvm_coalesced_mmio_ring *coalesced_mmio_ring;
+#endif
     int broken_set_mem_region;
     int migration_log;
     int vcpu_events;
@@ -200,6 +203,12 @@ int kvm_init_vcpu(CPUState *env)
         goto err;
     }
 
+#ifdef KVM_CAP_COALESCED_MMIO
+    if (s->coalesced_mmio && !s->coalesced_mmio_ring)
+        s->coalesced_mmio_ring = (void *) env->kvm_run +
+		s->coalesced_mmio * PAGE_SIZE;
+#endif
+
     ret = kvm_arch_init_vcpu(env);
     if (ret == 0) {
         qemu_register_reset(kvm_reset_vcpu, env);
@@ -466,10 +475,10 @@ int kvm_init(int smp_cpus)
         goto err;
     }
 
+    s->coalesced_mmio = 0;
 #ifdef KVM_CAP_COALESCED_MMIO
     s->coalesced_mmio = kvm_check_extension(s, KVM_CAP_COALESCED_MMIO);
-#else
-    s->coalesced_mmio = 0;
+    s->coalesced_mmio_ring = NULL;
 #endif
 
     s->broken_set_mem_region = 1;
@@ -544,14 +553,12 @@ static int kvm_handle_io(uint16_t port, void *data, int direction, int size,
     return 1;
 }
 
-static void kvm_run_coalesced_mmio(CPUState *env, struct kvm_run *run)
+void kvm_flush_coalesced_mmio_buffer(void)
 {
 #ifdef KVM_CAP_COALESCED_MMIO
     KVMState *s = kvm_state;
-    if (s->coalesced_mmio) {
-        struct kvm_coalesced_mmio_ring *ring;
-
-        ring = (void *)run + (s->coalesced_mmio * TARGET_PAGE_SIZE);
+    if (s->coalesced_mmio_ring) {
+        struct kvm_coalesced_mmio_ring *ring = s->coalesced_mmio_ring;
         while (ring->first != ring->last) {
             struct kvm_coalesced_mmio *ent;
 
@@ -609,7 +616,7 @@ int kvm_cpu_exec(CPUState *env)
             abort();
         }
 
-        kvm_run_coalesced_mmio(env, run);
+        kvm_flush_coalesced_mmio_buffer();
 
         ret = 0; /* exit loop */
         switch (run->exit_reason) {