memory: emulate ioeventfd

The ioeventfd mechanism is used by vhost, dataplane, and virtio-pci to
turn guest MMIO/PIO writes into eventfd file descriptor events.  This
allows arbitrary threads to be notified when the guest writes to a
specific MMIO/PIO address.

qtest and TCG do not support ioeventfd because memory writes are not
checked against registered ioeventfds in QEMU.  This patch implements
this in memory_region_dispatch_write() so qtest can use ioeventfd.

Also this patch fixes vhost aborting on some misconfigured old kernels
like 3.18.0 on ARM. It is possible to explicitly enable CONFIG_EVENTFD
in expert settings, while MMIO binding support in KVM will still be
missing.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Message-Id: <006e01d12377$0b9c2d40$22d487c0$@samsung.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/kvm-all.c b/kvm-all.c
index a45c92bd1c..ed707fe580 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1628,8 +1628,10 @@ static int kvm_init(MachineState *ms)
 
     kvm_state = s;
 
-    s->memory_listener.listener.eventfd_add = kvm_mem_ioeventfd_add;
-    s->memory_listener.listener.eventfd_del = kvm_mem_ioeventfd_del;
+    if (kvm_eventfds_allowed) {
+        s->memory_listener.listener.eventfd_add = kvm_mem_ioeventfd_add;
+        s->memory_listener.listener.eventfd_del = kvm_mem_ioeventfd_del;
+    }
     s->memory_listener.listener.coalesced_mmio_add = kvm_coalesce_mmio_region;
     s->memory_listener.listener.coalesced_mmio_del = kvm_uncoalesce_mmio_region;