[Qemu-devel] [PATCH 4/6] virtio-pci: use wildcard mmio eventfd for 1.0 notification cap

[Jason Wang <jasowang@redhat.com>]

We use data match eventfd for 1.0 notification currently. This could
be slow since software decoding is needed for mmio exit. To speed this
up, we can switch to use wild card mmio eventfd for 1.0 notification
since we can examine the queue index directly from the writing
address. KVM kernel module can utilize this by registering it to fast
mmio bus which could be as fast as pio on ept capable machine.

Lots of improvements were seen on a ept capable machine:

Guest RX:(TCP)
size/session/+throughput%/+cpu%/-+per cpu%/
64/1/+1.6807%/[-16.2421%]/[+21.3984%]/
64/2/+0.6091%/[-11.0187%]/[+13.0678%]/
64/4/+0.0553%/[-5.9768%]/[+6.4155%]/
64/8/+0.1206%/[-4.0057%]/[+4.2984%]/
256/1/-0.0031%/[-10.1166%]/[+11.2517%]/
256/2/-0.5058%/[-6.1656%]/+6.0317%]/
...

Guest TX:(TCP)
size/session/+throughput%/+cpu%/-+per cpu%/
64/1/[+18.9183%]/-0.2823%/[+19.2550%]/
64/2/[+13.5714%]/[+2.2675%]/[+11.0533%]/
64/4/[+13.1070%]/[+2.1817%]/[+10.6920%]/
64/8/[+13.0426%]/[+2.0887%]/[+10.7299%]/
256/1/[+36.2761%]/+6.3434%/[+28.1471%]/
...
1024/1/[+44.8873%]/+2.0811%/[+41.9335%]/
...
1024/4/+0.0228%/[-2.2044%]/[+2.2774%]/
...
16384/2/+0.0127%/[-5.0346%]/[+5.3148%]/
...
65535/1/[+0.0062%]/[-4.1183%]/[+4.3017%]/
65535/2/+0.0004%/[-4.2311%]/[+4.4185%]/
65535/4/+0.0107%/[-4.6106%]/[+4.8446%]/
65535/8/-0.0090%/[-5.5178%]/[+5.8306%]/

Latency:(TCP_RR)
size/session/+transaction rate%/+cpu%/-+per cpu%/
64/1/[+6.5248%]/[-9.2882%]/[+17.4322%]/
64/25/[+11.0854%]/[+0.8000%]/[+10.2038%]/
64/50/[+12.1076%]/[+2.4627%]/[+9.4131%]/
256/1/[+5.3677%]/[+10.5669%]/-4.7024%/
256/25/[+5.6402%]/-0.8962%/[+6.5955%]/
256/50/[+5.9685%]/[+1.7766%]/[+4.1188%]/
4096/1/+0.2508%/[-10.4941%]/[+12.0047%]/
4096/25/[+1.8533%]/-0.0273%/+1.8812%/
4096/50/[+1.2156%]/-1.4134%/+2.6667%/

Notes: data with '[]' is the one whose significance is greater than 95%.

Thanks Wenli Quan <wquan@redhat.com> for the benchmarking.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 hw/virtio/virtio-pci.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index d785623..fbd1f1f 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -226,8 +226,8 @@ static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
         }
         virtio_queue_set_host_notifier_fd_handler(vq, true, set_handler);
         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,
@@ -235,8 +235,8 @@ static int virtio_pci_set_host_notifier_internal(VirtIOPCIProxy *proxy,
         }
     } else {
         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,
-- 
2.1.4