[Qemu-devel] [RFC v9 18/27] virtio-blk: Call ioctl() directly instead of irqfd

[Stefan Hajnoczi]

Optimize for the MSI-X enabled and vector unmasked case where it is
possible to issue the KVM ioctl() directly instead of using irqfd.

This patch introduces a new virtio binding function which tries to
notify in a thread-safe way.  If this is not possible, the function
returns false.  Virtio block then knows to use irqfd as a fallback.
---
 hw/msix.c       |   17 +++++++++++++++++
 hw/msix.h       |    1 +
 hw/virtio-blk.c |   10 ++++++++--
 hw/virtio-pci.c |    8 ++++++++
 hw/virtio.c     |    9 +++++++++
 hw/virtio.h     |    3 +++
 6 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/hw/msix.c b/hw/msix.c
index 7955221..3308604 100644
--- a/hw/msix.c
+++ b/hw/msix.c
@@ -503,6 +503,23 @@ void msix_notify(PCIDevice *dev, unsigned vector)
     stl_le_phys(address, data);
 }
 
+bool msix_try_notify_from_thread(PCIDevice *dev, unsigned vector)
+{
+    if (unlikely(vector >= dev->msix_entries_nr || 
!dev->msix_entry_used[vector])) {
+        return false;
+    }
+    if (unlikely(msix_is_masked(dev, vector))) {
+        return false;
+    }
+#ifdef KVM_CAP_IRQCHIP
+    if (likely(kvm_enabled() && kvm_irqchip_in_kernel())) {
+        kvm_set_irq(dev->msix_irq_entries[vector].gsi, 1, NULL);
+        return true;
+    }
+#endif
+    return false;
+}
+
 void msix_reset(PCIDevice *dev)
 {
     if (!(dev->cap_present & QEMU_PCI_CAP_MSIX))
diff --git a/hw/msix.h b/hw/msix.h
index a8661e1..99fb08f 100644
--- a/hw/msix.h
+++ b/hw/msix.h
@@ -26,6 +26,7 @@ void msix_vector_unuse(PCIDevice *dev, unsigned vector);
 void msix_unuse_all_vectors(PCIDevice *dev);
 
 void msix_notify(PCIDevice *dev, unsigned vector);
+bool msix_try_notify_from_thread(PCIDevice *dev, unsigned vector);
 
 void msix_reset(PCIDevice *dev);
 
diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index bdff68a..efeffa0 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -82,6 +82,12 @@ static void virtio_blk_notify_guest(VirtIOBlock *s)
         !(s->vdev.guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY))))
                return;
 
+    /* Try to issue the ioctl() directly for speed */
+    if (likely(virtio_queue_try_notify_from_thread(s->vq))) {
+        return;
+    }
+
+    /* If the fast path didn't work, use irqfd */
     event_notifier_set(virtio_queue_get_guest_notifier(s->vq));
 }
 
@@ -263,7 +269,7 @@ static void data_plane_start(VirtIOBlock *s)
     vring_setup(&s->vring, &s->vdev, 0);
 
     /* Set up guest notifier (irq) */
-    if (s->vdev.binding->set_guest_notifier(s->vdev.binding_opaque, 0, true) 
!= 0) {
+    if (s->vdev.binding->set_guest_notifiers(s->vdev.binding_opaque, true) != 
0) {
         fprintf(stderr, "virtio-blk failed to set guest notifier, ensure 
-enable-kvm is set\n");
         exit(1);
     }
@@ -315,7 +321,7 @@ static void data_plane_stop(VirtIOBlock *s)
     event_poll_cleanup(&s->event_poll);
 
     /* Clean up guest notifier (irq) */
-    s->vdev.binding->set_guest_notifier(s->vdev.binding_opaque, 0, false);
+    s->vdev.binding->set_guest_notifiers(s->vdev.binding_opaque, false);
 }
 
 static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t val)
diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index f1e13af..03512b3 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -106,6 +106,13 @@ static void virtio_pci_notify(void *opaque, uint16_t 
vector)
         qemu_set_irq(proxy->pci_dev.irq[0], proxy->vdev->isr & 1);
 }
 
+static bool virtio_pci_try_notify_from_thread(void *opaque, uint16_t vector)
+{
+    VirtIOPCIProxy *proxy = opaque;
+    return msix_enabled(&proxy->pci_dev) &&
+           msix_try_notify_from_thread(&proxy->pci_dev, vector);
+}
+
 static void virtio_pci_save_config(void * opaque, QEMUFile *f)
 {
     VirtIOPCIProxy *proxy = opaque;
@@ -707,6 +714,7 @@ static void virtio_pci_vmstate_change(void *opaque, bool 
running)
 
 static const VirtIOBindings virtio_pci_bindings = {
     .notify = virtio_pci_notify,
+    .try_notify_from_thread = virtio_pci_try_notify_from_thread,
     .save_config = virtio_pci_save_config,
     .load_config = virtio_pci_load_config,
     .save_queue = virtio_pci_save_queue,
diff --git a/hw/virtio.c b/hw/virtio.c
index 064aecf..a1d1a8a 100644
--- a/hw/virtio.c
+++ b/hw/virtio.c
@@ -689,6 +689,15 @@ static inline int vring_need_event(uint16_t event, 
uint16_t new, uint16_t old)
        return (uint16_t)(new - event - 1) < (uint16_t)(new - old);
 }
 
+bool virtio_queue_try_notify_from_thread(VirtQueue *vq)
+{
+    VirtIODevice *vdev = vq->vdev;
+    if (likely(vdev->binding->try_notify_from_thread)) {
+        return vdev->binding->try_notify_from_thread(vdev->binding_opaque, 
vq->vector);
+    }
+    return false;
+}
+
 static bool vring_notify(VirtIODevice *vdev, VirtQueue *vq)
 {
     uint16_t old, new;
diff --git a/hw/virtio.h b/hw/virtio.h
index 400c092..2cdf2be 100644
--- a/hw/virtio.h
+++ b/hw/virtio.h
@@ -93,6 +93,7 @@ typedef struct VirtQueueElement
 
 typedef struct {
     void (*notify)(void * opaque, uint16_t vector);
+    bool (*try_notify_from_thread)(void * opaque, uint16_t vector);
     void (*save_config)(void * opaque, QEMUFile *f);
     void (*save_queue)(void * opaque, int n, QEMUFile *f);
     int (*load_config)(void * opaque, QEMUFile *f);
@@ -160,6 +161,8 @@ void virtio_cleanup(VirtIODevice *vdev);
 
 void virtio_notify_config(VirtIODevice *vdev);
 
+bool virtio_queue_try_notify_from_thread(VirtQueue *vq);
+
 void virtio_queue_set_notification(VirtQueue *vq, int enable);
 
 int virtio_queue_ready(VirtQueue *vq);
-- 
1.7.10.4