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Re: [RFC v2 11/13] vhost: Shadow virtqueue buffers forwarding


From: Jason Wang
Subject: Re: [RFC v2 11/13] vhost: Shadow virtqueue buffers forwarding
Date: Tue, 16 Mar 2021 16:15:03 +0800
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.16; rv:78.0) Gecko/20100101 Thunderbird/78.8.1


在 2021/3/16 上午3:48, Eugenio Pérez 写道:
Initial version of shadow virtqueue that actually forward buffers.

It reuses the VirtQueue code for the device part. The driver part is
based on Linux's virtio_ring driver, but with stripped functionality
and optimizations so it's easier to review.

These will be added in later commits.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
  hw/virtio/vhost-shadow-virtqueue.c | 212 +++++++++++++++++++++++++++--
  hw/virtio/vhost.c                  | 113 ++++++++++++++-
  2 files changed, 312 insertions(+), 13 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
b/hw/virtio/vhost-shadow-virtqueue.c
index 1460d1d5d1..68ed0f2740 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -9,6 +9,7 @@
#include "hw/virtio/vhost-shadow-virtqueue.h"
  #include "hw/virtio/vhost.h"
+#include "hw/virtio/virtio-access.h"
#include "standard-headers/linux/vhost_types.h" @@ -55,11 +56,96 @@ typedef struct VhostShadowVirtqueue {
      /* Virtio device */
      VirtIODevice *vdev;
+ /* Map for returning guest's descriptors */
+    VirtQueueElement **ring_id_maps;
+
+    /* Next head to expose to device */
+    uint16_t avail_idx_shadow;
+
+    /* Next free descriptor */
+    uint16_t free_head;
+
+    /* Last seen used idx */
+    uint16_t shadow_used_idx;
+
+    /* Next head to consume from device */
+    uint16_t used_idx;
+
      /* Descriptors copied from guest */
      vring_desc_t descs[];
  } VhostShadowVirtqueue;
-/* Forward guest notifications */
+static void vhost_vring_write_descs(VhostShadowVirtqueue *svq,
+                                    const struct iovec *iovec,
+                                    size_t num, bool more_descs, bool write)
+{
+    uint16_t i = svq->free_head, last = svq->free_head;
+    unsigned n;
+    uint16_t flags = write ? virtio_tswap16(svq->vdev, VRING_DESC_F_WRITE) : 0;
+    vring_desc_t *descs = svq->vring.desc;
+
+    if (num == 0) {
+        return;
+    }
+
+    for (n = 0; n < num; n++) {
+        if (more_descs || (n + 1 < num)) {
+            descs[i].flags = flags | virtio_tswap16(svq->vdev,
+                                                    VRING_DESC_F_NEXT);
+        } else {
+            descs[i].flags = flags;
+        }
+        descs[i].addr = virtio_tswap64(svq->vdev, (hwaddr)iovec[n].iov_base);


So unsing virtio_tswap() is probably not correct since we're talking with vhost backends which has its own endiness.

For vhost-vDPA, we can assume that it's a 1.0 device.


+        descs[i].len = virtio_tswap32(svq->vdev, iovec[n].iov_len);
+
+        last = i;
+        i = virtio_tswap16(svq->vdev, descs[i].next);
+    }
+
+    svq->free_head = virtio_tswap16(svq->vdev, descs[last].next);
+}
+
+static unsigned vhost_shadow_vq_add_split(VhostShadowVirtqueue *svq,
+                                          VirtQueueElement *elem)
+{
+    int head;
+    unsigned avail_idx;
+    vring_avail_t *avail = svq->vring.avail;
+
+    head = svq->free_head;
+
+    /* We need some descriptors here */
+    assert(elem->out_num || elem->in_num);
+
+    vhost_vring_write_descs(svq, elem->out_sg, elem->out_num,
+                            elem->in_num > 0, false);
+    vhost_vring_write_descs(svq, elem->in_sg, elem->in_num, false, true);
+
+    /*
+     * Put entry in available array (but don't update avail->idx until they
+     * do sync).
+     */
+    avail_idx = svq->avail_idx_shadow & (svq->vring.num - 1);
+    avail->ring[avail_idx] = virtio_tswap16(svq->vdev, head);
+    svq->avail_idx_shadow++;
+
+    /* Expose descriptors to device */
+    smp_wmb();
+    avail->idx = virtio_tswap16(svq->vdev, svq->avail_idx_shadow);
+
+    return head;
+
+}
+
+static void vhost_shadow_vq_add(VhostShadowVirtqueue *svq,
+                                VirtQueueElement *elem)
+{
+    unsigned qemu_head = vhost_shadow_vq_add_split(svq, elem);
+
+    svq->ring_id_maps[qemu_head] = elem;
+}
+
+/* Handle guest->device notifications */
  static void vhost_handle_guest_kick(EventNotifier *n)
  {
      VhostShadowVirtqueue *svq = container_of(n, VhostShadowVirtqueue,
@@ -69,7 +155,72 @@ static void vhost_handle_guest_kick(EventNotifier *n)
          return;
      }
- event_notifier_set(&svq->kick_notifier);
+    /* Make available as many buffers as possible */
+    do {
+        if (virtio_queue_get_notification(svq->vq)) {
+            /* No more notifications until process all available */
+            virtio_queue_set_notification(svq->vq, false);
+        }
+
+        while (true) {
+            VirtQueueElement *elem;
+            if (virtio_queue_full(svq->vq)) {
+                break;


So we've disabled guest notification. If buffer has been consumed, we need to retry the handle_guest_kick here. But I didn't find the code?


+            }
+
+            elem = virtqueue_pop(svq->vq, sizeof(*elem));
+            if (!elem) {
+                break;
+            }
+
+            vhost_shadow_vq_add(svq, elem);
+            event_notifier_set(&svq->kick_notifier);
+        }
+
+        virtio_queue_set_notification(svq->vq, true);
+    } while (!virtio_queue_empty(svq->vq));
+}
+
+static bool vhost_shadow_vq_more_used(VhostShadowVirtqueue *svq)
+{
+    if (svq->used_idx != svq->shadow_used_idx) {
+        return true;
+    }
+
+    /* Get used idx must not be reordered */
+    smp_rmb();
+    svq->shadow_used_idx = virtio_tswap16(svq->vdev, svq->vring.used->idx);
+
+    return svq->used_idx != svq->shadow_used_idx;
+}
+
+static VirtQueueElement *vhost_shadow_vq_get_buf(VhostShadowVirtqueue *svq)
+{
+    vring_desc_t *descs = svq->vring.desc;
+    const vring_used_t *used = svq->vring.used;
+    vring_used_elem_t used_elem;
+    uint16_t last_used;
+
+    if (!vhost_shadow_vq_more_used(svq)) {
+        return NULL;
+    }
+
+    last_used = svq->used_idx & (svq->vring.num - 1);
+    used_elem.id = virtio_tswap32(svq->vdev, used->ring[last_used].id);
+    used_elem.len = virtio_tswap32(svq->vdev, used->ring[last_used].len);
+
+    if (unlikely(used_elem.id >= svq->vring.num)) {
+        error_report("Device %s says index %u is available", svq->vdev->name,
+                     used_elem.id);
+        return NULL;
+    }
+
+    descs[used_elem.id].next = svq->free_head;
+    svq->free_head = used_elem.id;
+
+    svq->used_idx++;
+    svq->ring_id_maps[used_elem.id]->len = used_elem.len;
+    return g_steal_pointer(&svq->ring_id_maps[used_elem.id]);
  }
/* Forward vhost notifications */
@@ -78,6 +229,7 @@ static void 
vhost_shadow_vq_handle_call_no_test(EventNotifier *n)
      VhostShadowVirtqueue *svq = container_of(n, VhostShadowVirtqueue,
                                               call_notifier);
      EventNotifier *masked_notifier;
+    VirtQueue *vq = svq->vq;
/* Signal start of using masked notifier */
      qemu_event_reset(&svq->masked_notifier.is_free);
@@ -86,14 +238,29 @@ static void 
vhost_shadow_vq_handle_call_no_test(EventNotifier *n)
          qemu_event_set(&svq->masked_notifier.is_free);
      }
- if (!masked_notifier) {
-        unsigned n = virtio_get_queue_index(svq->vq);
-        virtio_queue_invalidate_signalled_used(svq->vdev, n);
-        virtio_notify_irqfd(svq->vdev, svq->vq);
-    } else if (!svq->masked_notifier.signaled) {
-        svq->masked_notifier.signaled = true;
-        event_notifier_set(svq->masked_notifier.n);
-    }
+    /* Make as many buffers as possible used. */
+    do {
+        unsigned i = 0;
+
+        /* TODO: Use VRING_AVAIL_F_NO_INTERRUPT */
+        while (true) {
+            g_autofree VirtQueueElement *elem = vhost_shadow_vq_get_buf(svq);
+            if (!elem) {
+                break;
+            }
+
+            assert(i < svq->vring.num);
+            virtqueue_fill(vq, elem, elem->len, i++);
+        }
+
+        virtqueue_flush(vq, i);
+        if (!masked_notifier) {
+            virtio_notify_irqfd(svq->vdev, svq->vq);
+        } else if (!svq->masked_notifier.signaled) {
+            svq->masked_notifier.signaled = true;
+            event_notifier_set(svq->masked_notifier.n);
+        }
+    } while (vhost_shadow_vq_more_used(svq));
if (masked_notifier) {
          /* Signal not using it anymore */
@@ -103,7 +270,6 @@ static void 
vhost_shadow_vq_handle_call_no_test(EventNotifier *n)
static void vhost_shadow_vq_handle_call(EventNotifier *n)
  {
-
      if (likely(event_notifier_test_and_clear(n))) {
          vhost_shadow_vq_handle_call_no_test(n);
      }
@@ -254,7 +420,11 @@ void vhost_shadow_vq_stop(struct vhost_dev *dev,
                            unsigned idx,
                            VhostShadowVirtqueue *svq)
  {
+    int i;
      int r = vhost_shadow_vq_restore_vdev_host_notifier(dev, idx, svq);
+
+    assert(!dev->shadow_vqs_enabled);
+
      if (unlikely(r < 0)) {
          error_report("Couldn't restore vq kick fd: %s", strerror(-r));
      }
@@ -272,6 +442,18 @@ void vhost_shadow_vq_stop(struct vhost_dev *dev,
      /* Restore vhost call */
      vhost_virtqueue_mask(dev, dev->vdev, dev->vq_index + idx,
                           dev->vqs[idx].notifier_is_masked);
+
+
+    for (i = 0; i < svq->vring.num; ++i) {
+        g_autofree VirtQueueElement *elem = svq->ring_id_maps[i];
+        /*
+         * Although the doc says we must unpop in order, it's ok to unpop
+         * everything.
+         */
+        if (elem) {
+            virtqueue_unpop(svq->vq, elem, elem->len);


Shouldn't we need to wait until all pending requests to be drained? Or we may end up duplicated requests?

Thanks


+        }
+    }
  }
/*
@@ -284,7 +466,7 @@ VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev 
*dev, int idx)
      unsigned num = virtio_queue_get_num(dev->vdev, vq_idx);
      size_t ring_size = vring_size(num, VRING_DESC_ALIGN_SIZE);
      g_autofree VhostShadowVirtqueue *svq = g_malloc0(sizeof(*svq) + 
ring_size);
-    int r;
+    int r, i;
r = event_notifier_init(&svq->kick_notifier, 0);
      if (r != 0) {
@@ -303,6 +485,11 @@ VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev 
*dev, int idx)
      vring_init(&svq->vring, num, svq->descs, VRING_DESC_ALIGN_SIZE);
      svq->vq = virtio_get_queue(dev->vdev, vq_idx);
      svq->vdev = dev->vdev;
+    for (i = 0; i < num - 1; i++) {
+        svq->descs[i].next = virtio_tswap16(dev->vdev, i + 1);
+    }
+
+    svq->ring_id_maps = g_new0(VirtQueueElement *, num);
      event_notifier_set_handler(&svq->call_notifier,
                                 vhost_shadow_vq_handle_call);
      qemu_event_init(&svq->masked_notifier.is_free, true);
@@ -324,5 +511,6 @@ void vhost_shadow_vq_free(VhostShadowVirtqueue *vq)
      event_notifier_cleanup(&vq->kick_notifier);
      event_notifier_set_handler(&vq->call_notifier, NULL);
      event_notifier_cleanup(&vq->call_notifier);
+    g_free(vq->ring_id_maps);
      g_free(vq);
  }
diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index eab3e334f2..a373999bc4 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1021,6 +1021,19 @@ int vhost_device_iotlb_miss(struct vhost_dev *dev, 
uint64_t iova, int write)
trace_vhost_iotlb_miss(dev, 1); + if (qatomic_load_acquire(&dev->shadow_vqs_enabled)) {
+        uaddr = iova;
+        len = 4096;
+        ret = vhost_backend_update_device_iotlb(dev, iova, uaddr, len,
+                                                IOMMU_RW);
+        if (ret) {
+            trace_vhost_iotlb_miss(dev, 2);
+            error_report("Fail to update device iotlb");
+        }
+
+        return ret;
+    }
+
      iotlb = address_space_get_iotlb_entry(dev->vdev->dma_as,
                                            iova, write,
                                            MEMTXATTRS_UNSPECIFIED);
@@ -1227,8 +1240,28 @@ static int vhost_sw_live_migration_stop(struct vhost_dev 
*dev)
      /* Can be read by vhost_virtqueue_mask, from vm exit */
      qatomic_store_release(&dev->shadow_vqs_enabled, false);
+ dev->vhost_ops->vhost_set_vring_enable(dev, false);
+    if (vhost_backend_invalidate_device_iotlb(dev, 0, -1ULL)) {
+        error_report("Fail to invalidate device iotlb");
+    }
+
      for (idx = 0; idx < dev->nvqs; ++idx) {
+        /*
+         * Update used ring information for IOTLB to work correctly,
+         * vhost-kernel code requires for this.
+         */
+        struct vhost_virtqueue *vq = dev->vqs + idx;
+        vhost_device_iotlb_miss(dev, vq->used_phys, true);
+
          vhost_shadow_vq_stop(dev, idx, dev->shadow_vqs[idx]);
+        vhost_virtqueue_start(dev, dev->vdev, &dev->vqs[idx],
+                              dev->vq_index + idx);
+    }
+
+    /* Enable guest's vq vring */
+    dev->vhost_ops->vhost_set_vring_enable(dev, true);
+
+    for (idx = 0; idx < dev->nvqs; ++idx) {
          vhost_shadow_vq_free(dev->shadow_vqs[idx]);
      }
@@ -1237,6 +1270,59 @@ static int vhost_sw_live_migration_stop(struct vhost_dev *dev)
      return 0;
  }
+/*
+ * Start shadow virtqueue in a given queue.
+ * In failure case, this function leaves queue working as regular vhost mode.
+ */
+static bool vhost_sw_live_migration_start_vq(struct vhost_dev *dev,
+                                             unsigned idx)
+{
+    struct vhost_vring_addr addr = {
+        .index = idx,
+    };
+    struct vhost_vring_state s = {
+        .index = idx,
+    };
+    int r;
+    bool ok;
+
+    vhost_virtqueue_stop(dev, dev->vdev, &dev->vqs[idx], dev->vq_index + idx);
+    ok = vhost_shadow_vq_start(dev, idx, dev->shadow_vqs[idx]);
+    if (unlikely(!ok)) {
+        return false;
+    }
+
+    /* From this point, vhost_virtqueue_start can reset these changes */
+    vhost_shadow_vq_get_vring_addr(dev->shadow_vqs[idx], &addr);
+    r = dev->vhost_ops->vhost_set_vring_addr(dev, &addr);
+    if (unlikely(r != 0)) {
+        VHOST_OPS_DEBUG("vhost_set_vring_addr for shadow vq failed");
+        goto err;
+    }
+
+    r = dev->vhost_ops->vhost_set_vring_base(dev, &s);
+    if (unlikely(r != 0)) {
+        VHOST_OPS_DEBUG("vhost_set_vring_base for shadow vq failed");
+        goto err;
+    }
+
+    /*
+     * Update used ring information for IOTLB to work correctly,
+     * vhost-kernel code requires for this.
+     */
+    r = vhost_device_iotlb_miss(dev, addr.used_user_addr, true);
+    if (unlikely(r != 0)) {
+        /* Debug message already printed */
+        goto err;
+    }
+
+    return true;
+
+err:
+    vhost_virtqueue_start(dev, dev->vdev, &dev->vqs[idx], dev->vq_index + idx);
+    return false;
+}
+
  static int vhost_sw_live_migration_start(struct vhost_dev *dev)
  {
      int idx, stop_idx;
@@ -1249,24 +1335,35 @@ static int vhost_sw_live_migration_start(struct 
vhost_dev *dev)
          }
      }
+ dev->vhost_ops->vhost_set_vring_enable(dev, false);
+    if (vhost_backend_invalidate_device_iotlb(dev, 0, -1ULL)) {
+        error_report("Fail to invalidate device iotlb");
+    }
+
      /* Can be read by vhost_virtqueue_mask, from vm exit */
      qatomic_store_release(&dev->shadow_vqs_enabled, true);
      for (idx = 0; idx < dev->nvqs; ++idx) {
-        bool ok = vhost_shadow_vq_start(dev, idx, dev->shadow_vqs[idx]);
+        bool ok = vhost_sw_live_migration_start_vq(dev, idx);
          if (unlikely(!ok)) {
              goto err_start;
          }
      }
+ /* Enable shadow vq vring */
+    dev->vhost_ops->vhost_set_vring_enable(dev, true);
      return 0;
err_start:
      qatomic_store_release(&dev->shadow_vqs_enabled, false);
      for (stop_idx = 0; stop_idx < idx; stop_idx++) {
          vhost_shadow_vq_stop(dev, idx, dev->shadow_vqs[stop_idx]);
+        vhost_virtqueue_start(dev, dev->vdev, &dev->vqs[idx],
+                              dev->vq_index + stop_idx);
      }
err_new:
+    /* Enable guest's vring */
+    dev->vhost_ops->vhost_set_vring_enable(dev, true);
      for (idx = 0; idx < dev->nvqs; ++idx) {
          vhost_shadow_vq_free(dev->shadow_vqs[idx]);
      }
@@ -1970,6 +2067,20 @@ void qmp_x_vhost_enable_shadow_vq(const char *name, bool 
enable, Error **errp)
if (!hdev->started) {
              err_cause = "Device is not started";
+        } else if (!vhost_dev_has_iommu(hdev)) {
+            err_cause = "Does not support iommu";
+        } else if (hdev->acked_features & BIT_ULL(VIRTIO_F_RING_PACKED)) {
+            err_cause = "Is packed";
+        } else if (hdev->acked_features & BIT_ULL(VIRTIO_RING_F_EVENT_IDX)) {
+            err_cause = "Have event idx";
+        } else if (hdev->acked_features &
+                   BIT_ULL(VIRTIO_RING_F_INDIRECT_DESC)) {
+            err_cause = "Supports indirect descriptors";
+        } else if (!hdev->vhost_ops->vhost_set_vring_enable) {
+            err_cause = "Cannot pause device";
+        }
+
+        if (err_cause) {
              goto err;
          }




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