This patch adds support for the VIRTIO_RING_F_EVENT_IDX feature for
interrupt mitigation. virtio-blk doesn't do anything fancy with it so
we may not see a performance improvement. This patch will allow newer
guest kernels to run successfully.
Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxxxxxxxxxx>
---
hw/dataplane/vring.h | 65 ++++++++++++++++++++++++++++++++++++++++----------
hw/virtio-blk.c | 16 ++++++-------
2 files changed, 60 insertions(+), 21 deletions(-)
diff --git a/hw/dataplane/vring.h b/hw/dataplane/vring.h
index bbf8c86..d939a22 100644
--- a/hw/dataplane/vring.h
+++ b/hw/dataplane/vring.h
@@ -14,6 +14,8 @@ typedef struct {
struct vring vr; /* virtqueue vring mapped to host memory */
__u16 last_avail_idx; /* last processed avail ring index */
__u16 last_used_idx; /* last processed used ring index */
+ uint16_t signalled_used; /* EVENT_IDX state */
+ bool signalled_used_valid;
} Vring;
static inline unsigned int vring_get_num(Vring *vring)
@@ -63,6 +65,8 @@ static void vring_setup(Vring *vring, VirtIODevice *vdev, int n)
vring->last_avail_idx = 0;
vring->last_used_idx = 0;
+ vring->signalled_used = 0;
+ vring->signalled_used_valid = false;
fprintf(stderr, "vring physical=%#lx desc=%p avail=%p used=%p\n",
(unsigned long)virtio_queue_get_ring_addr(vdev, n),
@@ -75,21 +79,48 @@ static bool vring_more_avail(Vring *vring)
return vring->vr.avail->idx != vring->last_avail_idx;
}
-/* Hint to disable guest->host notifies */
-static void vring_disable_cb(Vring *vring)
+/* Toggle guest->host notifies */
+static void vring_set_notification(VirtIODevice *vdev, Vring *vring, bool enable)
{
- vring->vr.used->flags |= VRING_USED_F_NO_NOTIFY;
+ if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
+ if (enable) {
+ vring_avail_event(&vring->vr) = vring->vr.avail->idx;
+ }
+ } else if (enable) {
+ vring->vr.used->flags &= ~VRING_USED_F_NO_NOTIFY;
+ } else {
+ vring->vr.used->flags |= VRING_USED_F_NO_NOTIFY;
+ }
}
-/* Re-enable guest->host notifies
- *
- * Returns false if there are more descriptors in the ring.
- */
-static bool vring_enable_cb(Vring *vring)
+/* This is stolen from linux/drivers/vhost/vhost.c:vhost_notify() */
+static bool vring_should_notify(VirtIODevice *vdev, Vring *vring)
{
- vring->vr.used->flags &= ~VRING_USED_F_NO_NOTIFY;
- __sync_synchronize(); /* mb() */
- return !vring_more_avail(vring);
+ uint16_t old, new;
+ bool v;
+ /* Flush out used index updates. This is paired
+ * with the barrier that the Guest executes when enabling
+ * interrupts. */
+ __sync_synchronize(); /* smp_mb() */
+
+ if ((vdev->guest_features & VIRTIO_F_NOTIFY_ON_EMPTY) &&
+ unlikely(vring->vr.avail->idx == vring->last_avail_idx)) {
+ return true;
+ }
+
+ if (!(vdev->guest_features & VIRTIO_RING_F_EVENT_IDX)) {
+ return !(vring->vr.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
+ }
+ old = vring->signalled_used;
+ v = vring->signalled_used_valid;
+ new = vring->signalled_used = vring->last_used_idx;
+ vring->signalled_used_valid = true;
+
+ if (unlikely(!v)) {
+ return true;
+ }
+
+ return vring_need_event(vring_used_event(&vring->vr), new, old);
}
/* This is stolen from linux-2.6/drivers/vhost/vhost.c. */
@@ -178,7 +209,7 @@ static bool get_indirect(Vring *vring,
*
* Stolen from linux-2.6/drivers/vhost/vhost.c.
*/
-static int vring_pop(Vring *vring,
+static int vring_pop(VirtIODevice *vdev, Vring *vring,
struct iovec iov[], struct iovec *iov_end,
unsigned int *out_num, unsigned int *in_num)
{
@@ -214,6 +245,10 @@ static int vring_pop(Vring *vring,
exit(1);
}
+ if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
+ vring_avail_event(&vring->vr) = vring->vr.avail->idx;
+ }
+
/* When we start there are none of either input nor output. */
*out_num = *in_num = 0;
@@ -279,6 +314,7 @@ static int vring_pop(Vring *vring,
static void vring_push(Vring *vring, unsigned int head, int len)
{
struct vring_used_elem *used;
+ uint16_t new;
/* The virtqueue contains a ring of used buffers. Get a pointer to the
* next entry in that used ring. */
@@ -289,7 +325,10 @@ static void vring_push(Vring *vring, unsigned int head, int len)
/* Make sure buffer is written before we update index. */
__sync_synchronize(); /* smp_wmb() */
- vring->vr.used->idx = ++vring->last_used_idx;
+ new = vring->vr.used->idx = ++vring->last_used_idx;
+ if (unlikely((int16_t)(new - vring->signalled_used) < (uint16_t)1)) {
+ vring->signalled_used_valid = false;
+ }
}
#endif /* VRING_H */
diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index cff2298..a3e3d8c 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -96,11 +96,9 @@ static int get_raw_posix_fd_hack(VirtIOBlock *s)
/* Raise an interrupt to signal guest, if necessary */
static void virtio_blk_notify_guest(VirtIOBlock *s)
{
- /* Always notify when queue is empty (when feature acknowledge) */
- if ((s->vring.vr.avail->flags & VRING_AVAIL_F_NO_INTERRUPT) &&
- (s->vring.vr.avail->idx != s->vring.last_avail_idx ||
- !(s->vdev.guest_features & (1 << VIRTIO_F_NOTIFY_ON_EMPTY))))
- return;
+ if (!vring_should_notify(&s->vdev, &s->vring)) {
+ return;
+ }
/* Try to issue the ioctl() directly for speed */
if (likely(virtio_queue_try_notify_from_thread(s->vq))) {
@@ -307,10 +305,10 @@ static bool handle_notify(EventHandler *handler)
for (;;) {
/* Disable guest->host notifies to avoid unnecessary vmexits */
- vring_disable_cb(&s->vring);
+ vring_set_notification(&s->vdev, &s->vring, false);
for (;;) {
- head = vring_pop(&s->vring, iov, end, &out_num, &in_num);
+ head = vring_pop(&s->vdev, &s->vring, iov, end, &out_num, &in_num);
if (head < 0) {
break; /* no more requests */
}
@@ -327,7 +325,9 @@ static bool handle_notify(EventHandler *handler)
/* Re-enable guest->host notifies and stop processing the vring.
* But if the guest has snuck in more descriptors, keep processing.
*/
- if (likely(vring_enable_cb(&s->vring))) {
+ vring_set_notification(&s->vdev, &s->vring, true);
+ __sync_synchronize(); /* smp_mb() */
+ if (!vring_more_avail(&s->vring)) {
break;
}
} else { /* head == -ENOBUFS, cannot continue since iovecs[] is depleted */
--
1.7.10.4
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