vhost: support event index

Support the new event index feature. When acked,
utilize it to reduce the # of interrupts sent to the guest.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 104 insertions, 34 deletions

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 7aa4eea930f1..ea966b356352 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -37,6 +37,9 @@ enum {
         VHOST_MEMORY_F_LOG = 0x1,
 };
 
+#define vhost_used_event(vq) ((u16 __user *)&vq->avail->ring[vq->num])
+#define vhost_avail_event(vq) ((u16 __user *)&vq->used->ring[vq->num])
+
 static void vhost_poll_func(struct file *file, wait_queue_head_t *wqh,
                             poll_table *pt)
 {
@@ -161,6 +164,8 @@ static void vhost_vq_reset(struct vhost_dev *dev,
         vq->last_avail_idx = 0;
         vq->avail_idx = 0;
         vq->last_used_idx = 0;
+        vq->signalled_used = 0;
+        vq->signalled_used_valid = false;
         vq->used_flags = 0;
         vq->log_used = false;
         vq->log_addr = -1ull;
@@ -489,16 +494,17 @@ static int memory_access_ok(struct vhost_dev *d, struct vhost_memory *mem,
         return 1;
 }
 
-static int vq_access_ok(unsigned int num,
+static int vq_access_ok(struct vhost_dev *d, unsigned int num,
                         struct vring_desc __user *desc,
                         struct vring_avail __user *avail,
                         struct vring_used __user *used)
 {
+        size_t s = vhost_has_feature(d, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
         return access_ok(VERIFY_READ, desc, num * sizeof *desc) &&
                access_ok(VERIFY_READ, avail,
-                         sizeof *avail + num * sizeof *avail->ring) &&
+                         sizeof *avail + num * sizeof *avail->ring + s) &&
                access_ok(VERIFY_WRITE, used,
-                        sizeof *used + num * sizeof *used->ring);
+                        sizeof *used + num * sizeof *used->ring + s);
 }
 
 /* Can we log writes? */
@@ -514,9 +520,11 @@ int vhost_log_access_ok(struct vhost_dev *dev)
 
 /* Verify access for write logging. */
 /* Caller should have vq mutex and device mutex */
-static int vq_log_access_ok(struct vhost_virtqueue *vq, void __user *log_base)
+static int vq_log_access_ok(struct vhost_dev *d, struct vhost_virtqueue *vq,
+                            void __user *log_base)
 {
         struct vhost_memory *mp;
+        size_t s = vhost_has_feature(d, VIRTIO_RING_F_EVENT_IDX) ? 2 : 0;
 
         mp = rcu_dereference_protected(vq->dev->memory,
                                        lockdep_is_held(&vq->mutex));
@@ -524,15 +532,15 @@ static int vq_log_access_ok(struct vhost_virtqueue *vq, void __user *log_base)
                             vhost_has_feature(vq->dev, VHOST_F_LOG_ALL)) &&
                 (!vq->log_used || log_access_ok(log_base, vq->log_addr,
                                         sizeof *vq->used +
-                                        vq->num * sizeof *vq->used->ring));
+                                        vq->num * sizeof *vq->used->ring + s));
 }
 
 /* Can we start vq? */
 /* Caller should have vq mutex and device mutex */
 int vhost_vq_access_ok(struct vhost_virtqueue *vq)
 {
-        return vq_access_ok(vq->num, vq->desc, vq->avail, vq->used) &&
-                vq_log_access_ok(vq, vq->log_base);
+        return vq_access_ok(vq->dev, vq->num, vq->desc, vq->avail, vq->used) &&
+                vq_log_access_ok(vq->dev, vq, vq->log_base);
 }
 
 static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
@@ -577,6 +585,7 @@ static int init_used(struct vhost_virtqueue *vq,
 
         if (r)
                 return r;
+        vq->signalled_used_valid = false;
         return get_user(vq->last_used_idx, &used->idx);
 }
 
@@ -674,7 +683,7 @@ static long vhost_set_vring(struct vhost_dev *d, int ioctl, void __user *argp)
                  * If it is not, we don't as size might not have been setup.
                  * We will verify when backend is configured. */
                 if (vq->private_data) {
-                        if (!vq_access_ok(vq->num,
+                        if (!vq_access_ok(d, vq->num,
                                 (void __user *)(unsigned long)a.desc_user_addr,
                                 (void __user *)(unsigned long)a.avail_user_addr,
                                 (void __user *)(unsigned long)a.used_user_addr)) {
@@ -818,7 +827,7 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
                         vq = d->vqs + i;
                         mutex_lock(&vq->mutex);
                         /* If ring is inactive, will check when it's enabled. */
-                        if (vq->private_data && !vq_log_access_ok(vq, base))
+                        if (vq->private_data && !vq_log_access_ok(d, vq, base))
                                 r = -EFAULT;
                         else
                                 vq->log_base = base;
@@ -1219,6 +1228,10 @@ int vhost_get_vq_desc(struct vhost_dev *dev, struct vhost_virtqueue *vq,
 
         /* On success, increment avail index. */
         vq->last_avail_idx++;
+
+        /* Assume notifications from guest are disabled at this point,
+         * if they aren't we would need to update avail_event index. */
+        BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
         return head;
 }
 
@@ -1267,6 +1280,12 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
                         eventfd_signal(vq->log_ctx, 1);
         }
         vq->last_used_idx++;
+        /* If the driver never bothers to signal in a very long while,
+         * used index might wrap around. If that happens, invalidate
+         * signalled_used index we stored. TODO: make sure driver
+         * signals at least once in 2^16 and remove this. */
+        if (unlikely(vq->last_used_idx == vq->signalled_used))
+                vq->signalled_used_valid = false;
         return 0;
 }
 
@@ -1275,6 +1294,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
                             unsigned count)
 {
         struct vring_used_elem __user *used;
+        u16 old, new;
         int start;
 
         start = vq->last_used_idx % vq->num;
@@ -1292,7 +1312,14 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
                            ((void __user *)used - (void __user *)vq->used),
                           count * sizeof *used);
         }
-        vq->last_used_idx += count;
+        old = vq->last_used_idx;
+        new = (vq->last_used_idx += count);
+        /* If the driver never bothers to signal in a very long while,
+         * used index might wrap around. If that happens, invalidate
+         * signalled_used index we stored. TODO: make sure driver
+         * signals at least once in 2^16 and remove this. */
+        if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
+                vq->signalled_used_valid = false;
         return 0;
 }
 
@@ -1331,29 +1358,47 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
         return r;
 }
 
-/* This actually signals the guest, using eventfd. */
-void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
+static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
-        __u16 flags;
-
+        __u16 old, new, event;
+        bool v;
         /* Flush out used index updates. This is paired
          * with the barrier that the Guest executes when enabling
          * interrupts. */
         smp_mb();
 
-        if (__get_user(flags, &vq->avail->flags)) {
-                vq_err(vq, "Failed to get flags");
-                return;
+        if (vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY) &&
+            unlikely(vq->avail_idx == vq->last_avail_idx))
+                return true;
+
+        if (!vhost_has_feature(dev, VIRTIO_RING_F_EVENT_IDX)) {
+                __u16 flags;
+                if (__get_user(flags, &vq->avail->flags)) {
+                        vq_err(vq, "Failed to get flags");
+                        return true;
+                }
+                return !(flags & VRING_AVAIL_F_NO_INTERRUPT);
         }
+        old = vq->signalled_used;
+        v = vq->signalled_used_valid;
+        new = vq->signalled_used = vq->last_used_idx;
+        vq->signalled_used_valid = true;
 
-        /* If they don't want an interrupt, don't signal, unless empty. */
-        if ((flags & VRING_AVAIL_F_NO_INTERRUPT) &&
-            (vq->avail_idx != vq->last_avail_idx ||
-             !vhost_has_feature(dev, VIRTIO_F_NOTIFY_ON_EMPTY)))
-                return;
+        if (unlikely(!v))
+                return true;
 
+        if (get_user(event, vhost_used_event(vq))) {
+                vq_err(vq, "Failed to get used event idx");
+                return true;
+        }
+        return vring_need_event(event, new, old);
+}
+
+/* This actually signals the guest, using eventfd. */
+void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq)
+{
         /* Signal the Guest tell them we used something up. */
-        if (vq->call_ctx)
+        if (vq->call_ctx && vhost_notify(dev, vq))
                 eventfd_signal(vq->call_ctx, 1);
 }
 
@@ -1376,7 +1421,7 @@ void vhost_add_used_and_signal_n(struct vhost_dev *dev,
 }
 
 /* OK, now we need to know about added descriptors. */
-bool vhost_enable_notify(struct vhost_virtqueue *vq)
+bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
         u16 avail_idx;
         int r;
@@ -1384,11 +1429,34 @@ bool vhost_enable_notify(struct vhost_virtqueue *vq)
         if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
                 return false;
         vq->used_flags &= ~VRING_USED_F_NO_NOTIFY;
-        r = put_user(vq->used_flags, &vq->used->flags);
-        if (r) {
-                vq_err(vq, "Failed to enable notification at %p: %d\n",
-                       &vq->used->flags, r);
-                return false;
+        if (!vhost_has_feature(dev, VIRTIO_RING_F_EVENT_IDX)) {
+                r = put_user(vq->used_flags, &vq->used->flags);
+                if (r) {
+                        vq_err(vq, "Failed to enable notification at %p: %d\n",
+                               &vq->used->flags, r);
+                        return false;
+                }
+        } else {
+                r = put_user(vq->avail_idx, vhost_avail_event(vq));
+                if (r) {
+                        vq_err(vq, "Failed to update avail event index at %p: %d\n",
+                               vhost_avail_event(vq), r);
+                        return false;
+                }
+        }
+        if (unlikely(vq->log_used)) {
+                void __user *used;
+                /* Make sure data is seen before log. */
+                smp_wmb();
+                used = vhost_has_feature(dev, VIRTIO_RING_F_EVENT_IDX) ?
+                        &vq->used->flags : vhost_avail_event(vq);
+                /* Log used flags or event index entry write. Both are 16 bit
+                 * fields. */
+                log_write(vq->log_base, vq->log_addr +
+                           (used - (void __user *)vq->used),
+                          sizeof(u16));
+                if (vq->log_ctx)
+                        eventfd_signal(vq->log_ctx, 1);
         }
         /* They could have slipped one in as we were doing that: make
          * sure it's written, then check again. */
@@ -1404,15 +1472,17 @@ bool vhost_enable_notify(struct vhost_virtqueue *vq)
 }
 
 /* We don't need to be notified again. */
-void vhost_disable_notify(struct vhost_virtqueue *vq)
+void vhost_disable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
         int r;
 
         if (vq->used_flags & VRING_USED_F_NO_NOTIFY)
                 return;
         vq->used_flags |= VRING_USED_F_NO_NOTIFY;
-        r = put_user(vq->used_flags, &vq->used->flags);
-        if (r)
-                vq_err(vq, "Failed to enable notification at %p: %d\n",
-                       &vq->used->flags, r);
+        if (!vhost_has_feature(dev, VIRTIO_RING_F_EVENT_IDX)) {
+                r = put_user(vq->used_flags, &vq->used->flags);
+                if (r)
+                        vq_err(vq, "Failed to enable notification at %p: %d\n",
+                               &vq->used->flags, r);
+        }
 }