KVM: Coalesced MMIO support

MMIO exits are more expensive in KVM or Xen than in QEMU because they 
involve, at least, privilege transitions.  However, MMIO write 
operations can be effectively batched if those writes do not have side 
effects.

Good examples of this include VGA pixel operations when in a planar 
mode.  As it turns out, we can get a nice boost in other areas too.  
Laurent mentioned a 9.7% performance boost in iperf with the coalesced 
MMIO changes for the e1000 when he originally posted this work for KVM.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>



git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5961 c046a42c-6fe2-441c-8c8c-71466251a162

1 files changed, 72 insertions, 0 deletions

diff --git a/kvm-all.c b/kvm-all.c
index 8615bf6b27..dad80df581 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -24,6 +24,9 @@
 #include "sysemu.h"
 #include "kvm.h"
 
+/* KVM uses PAGE_SIZE in it's definition of COALESCED_MMIO_MAX */
+#define PAGE_SIZE TARGET_PAGE_SIZE
+
 //#define DEBUG_KVM
 
 #ifdef DEBUG_KVM
@@ -52,6 +55,7 @@ struct KVMState
     KVMSlot slots[32];
     int fd;
     int vmfd;
+    int coalesced_mmio;
 };
 
 static KVMState *kvm_state;
@@ -228,6 +232,44 @@ out:
     qemu_free(d.dirty_bitmap);
 }
 
+int kvm_coalesce_mmio_region(target_phys_addr_t start, ram_addr_t size)
+{
+    int ret = -ENOSYS;
+#ifdef KVM_CAP_COALESCED_MMIO
+    KVMState *s = kvm_state;
+
+    if (s->coalesced_mmio) {
+        struct kvm_coalesced_mmio_zone zone;
+
+        zone.addr = start;
+        zone.size = size;
+
+        ret = kvm_vm_ioctl(s, KVM_REGISTER_COALESCED_MMIO, &zone);
+    }
+#endif
+
+    return ret;
+}
+
+int kvm_uncoalesce_mmio_region(target_phys_addr_t start, ram_addr_t size)
+{
+    int ret = -ENOSYS;
+#ifdef KVM_CAP_COALESCED_MMIO
+    KVMState *s = kvm_state;
+
+    if (s->coalesced_mmio) {
+        struct kvm_coalesced_mmio_zone zone;
+
+        zone.addr = start;
+        zone.size = size;
+
+        ret = kvm_vm_ioctl(s, KVM_UNREGISTER_COALESCED_MMIO, &zone);
+    }
+#endif
+
+    return ret;
+}
+
 int kvm_init(int smp_cpus)
 {
     KVMState *s;
@@ -298,6 +340,13 @@ int kvm_init(int smp_cpus)
         goto err;
     }
 
+    s->coalesced_mmio = 0;
+#ifdef KVM_CAP_COALESCED_MMIO
+    ret = kvm_ioctl(s, KVM_CHECK_EXTENSION, KVM_CAP_COALESCED_MMIO);
+    if (ret > 0)
+        s->coalesced_mmio = ret;
+#endif
+
     ret = kvm_arch_init(s, smp_cpus);
     if (ret < 0)
         goto err;
@@ -357,6 +406,27 @@ static int kvm_handle_io(CPUState *env, uint16_t port, void *data,
     return 1;
 }
 
+static void kvm_run_coalesced_mmio(CPUState *env, struct kvm_run *run)
+{
+#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);
+        while (ring->first != ring->last) {
+            struct kvm_coalesced_mmio *ent;
+
+            ent = &ring->coalesced_mmio[ring->first];
+
+            cpu_physical_memory_write(ent->phys_addr, ent->data, ent->len);
+            /* FIXME smp_wmb() */
+            ring->first = (ring->first + 1) % KVM_COALESCED_MMIO_MAX;
+        }
+    }
+#endif
+}
+
 int kvm_cpu_exec(CPUState *env)
 {
     struct kvm_run *run = env->kvm_run;
@@ -387,6 +457,8 @@ int kvm_cpu_exec(CPUState *env)
             abort();
         }
 
+        kvm_run_coalesced_mmio(env, run);
+
         ret = 0; /* exit loop */
         switch (run->exit_reason) {
         case KVM_EXIT_IO: