[Qemu-devel] [PATCH] vhost: force vhost off for non-MSI guests

[Michael S. Tsirkin]

When MSI is off, each interrupt needs to be bounced through the io
thread when it's set/cleared, so vhost-net causes more context switches and
higher CPU utilization than userspace virtio which handles networking in
the same thread.

We'll need to fix this by adding level irq support in kvm irqfd,
for now disable vhost-net in these configurations.

Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---

I need to report some error from virtio-pci
that would be handled specially (disable but don't
report an error) so I wanted one that's never likely to be used by a
userspace ioctl. I selected ERANGE but it'd
be easy to switch to something else. Comments?

 hw/vhost.c      |    4 +++-
 hw/virtio-net.c |    6 ++++--
 hw/virtio-pci.c |    3 +++
 hw/virtio.h     |    2 ++
 4 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/hw/vhost.c b/hw/vhost.c
index 1d09ed0..c79765a 100644
--- a/hw/vhost.c
+++ b/hw/vhost.c
@@ -649,7 +649,9 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice 
*vdev)
 
     r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
     if (r < 0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+       if (r != -EVIRTIO_DISABLED) {
+               fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+       }
         goto fail_notifiers;
     }
 
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index ccb3e63..5de3fee 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t 
status)
     if (!n->vhost_started) {
         int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), &n->vdev);
         if (r < 0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
         } else {
             n->vhost_started = 1;
         }
diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index dd8887a..dbf4be0 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -628,6 +628,9 @@ static int virtio_pci_set_guest_notifier(void *opaque, int 
n, bool assign)
     EventNotifier *notifier = virtio_queue_get_guest_notifier(vq);
 
     if (assign) {
+        if (!msix_enabled(&proxy->pci_dev)) {
+            return -EVIRTIO_DISABLED;
+        }
         int r = event_notifier_init(notifier, 0);
         if (r < 0) {
             return r;
diff --git a/hw/virtio.h b/hw/virtio.h
index d8546d5..53bbdba 100644
--- a/hw/virtio.h
+++ b/hw/virtio.h
@@ -98,6 +98,8 @@ typedef struct {
     void (*vmstate_change)(void * opaque, bool running);
 } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
 #define VIRTIO_PCI_QUEUE_MAX 64
 
 #define VIRTIO_NO_VECTOR 0xffff
-- 
1.7.3.2.91.g446ac