Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses

[Jason Wang <jasowang@redhat.com>]

On 08/31/2015 03:29 PM, Michael S. Tsirkin wrote:
>>>>> Thinking more about this, invoking the 0-length write after
>>>>> > >> > the != 0 length one would be better: it would mean we only
>>>>> > >> > handle the userspace MMIO like this.
>>> > > Right.
>>> > >
>> > 
>> > Using current unittest. This patch is about 2.9% slower than before, and
>> > invoking 0-length write after is still 1.1% slower (mmio-datamatch-eventfd).
>> > 
>> > /patch/result/-+%/
>> > /base/2957/0/
>> > /V3/3043/+2.9%/
>> > /V3+invoking != 0 length first/2990/+1.1%/
>> > 
>> > So looks like the best method is not searching KVM_FAST_MMIO_BUS during
>> > KVM_MMIO_BUS. Instead, let userspace to register both datamatch and
>> > wildcard in this case. Does this sound good to you?
> No - we can't change userspace.

Actually, the change was as simple as following. So I don't get the
reason why.

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 9935029..42ee986 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -288,6 +288,8 @@ static int
virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
         if (modern) {
             memory_region_add_eventfd(modern_mr, modern_addr, 2,
                                       true, n, notifier);
+            memory_region_add_eventfd(modern_mr, modern_addr, 0,
+                                      false, n, notifier);
         }
         if (legacy) {
             memory_region_add_eventfd(legacy_mr, legacy_addr, 2,
@@ -297,6 +299,8 @@ static int
virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
         if (modern) {
             memory_region_del_eventfd(modern_mr, modern_addr, 2,
                                       true, n, notifier);
+            memory_region_del_eventfd(modern_mr, modern_addr, 0,
+                                      false, n, notifier);
         }
         if (legacy) {
             memory_region_del_eventfd(legacy_mr, legacy_addr, 2,