Re: [RFC 09/10] vhost: Route guest->host notification through shadow virtqueue

On 2021/2/2 下午6:08, Eugenio Perez Martin wrote:

On Mon, Feb 1, 2021 at 7:29 AM Jason Wang [off-list ref] wrote:

quoted

On 2021/1/30 上午4:54, Eugenio Pérez wrote:

quoted

Shadow virtqueue notifications forwarding is disabled when vhost_dev
stops.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
   hw/virtio/vhost-shadow-virtqueue.h |   5 ++
   include/hw/virtio/vhost.h          |   4 +
   hw/virtio/vhost-shadow-virtqueue.c | 123 +++++++++++++++++++++++++-
   hw/virtio/vhost.c                  | 135 ++++++++++++++++++++++++++++-
   4 files changed, 264 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
index 6cc18d6acb..466f8ae595 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h

@@ -17,6 +17,11 @@

   typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;

+bool vhost_shadow_vq_start_rcu(struct vhost_dev *dev,
+                               VhostShadowVirtqueue *svq);
+void vhost_shadow_vq_stop_rcu(struct vhost_dev *dev,
+                              VhostShadowVirtqueue *svq);
+
   VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev *dev, int idx);

   void vhost_shadow_vq_free(VhostShadowVirtqueue *vq);

diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index 2be782cefd..732a4b2a2b 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h

@@ -55,6 +55,8 @@ struct vhost_iommu {
       QLIST_ENTRY(vhost_iommu) iommu_next;
   };

+typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
+
   typedef struct VhostDevConfigOps {
       /* Vhost device config space changed callback
        */

@@ -83,7 +85,9 @@ struct vhost_dev {
       uint64_t backend_cap;
       bool started;
       bool log_enabled;
+    bool sw_lm_enabled;
       uint64_t log_size;
+    VhostShadowVirtqueue **shadow_vqs;
       Error *migration_blocker;
       const VhostOps *vhost_ops;
       void *opaque;

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index c0c967a7c5..908c36c66d 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c

@@ -8,15 +8,129 @@
    */

   #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"
+#include "standard-headers/linux/virtio_ring.h"

   #include "qemu/error-report.h"
-#include "qemu/event_notifier.h"
+#include "qemu/main-loop.h"

   typedef struct VhostShadowVirtqueue {
       EventNotifier kick_notifier;
       EventNotifier call_notifier;
+    const struct vhost_virtqueue *hvq;
+    VirtIODevice *vdev;
+    VirtQueue *vq;
   } VhostShadowVirtqueue;

So instead of doing things at virtio level, how about do the shadow
stuffs at vhost level?

It works like:

virtio -> [shadow vhost backend] -> vhost backend

Then the QMP is used to plug the shadow vhost backend in the middle or not.

It looks kind of easier since we don't need to deal with virtqueue
handlers etc.. Instead, we just need to deal with eventfd stuffs:

When shadow vhost mode is enabled, we just intercept the host_notifiers
and guest_notifiers. When it was disabled, we just pass the host/guest
notifiers to the real vhost backends?

Hi Jason.

Sure we can try that model, but it seems to me that it comes with a
different set of problems.

For example, there are code in vhost.c that checks if implementations
are available in vhost_ops, like:

if (dev->vhost_ops->vhost_vq_get_addr) {
         r = dev->vhost_ops->vhost_vq_get_addr(dev, &addr, vq);
         ...
}

I can count 14 of these, checking:

dev->vhost_ops->vhost_backend_can_merge
dev->vhost_ops->vhost_backend_mem_section_filter
dev->vhost_ops->vhost_force_iommu
dev->vhost_ops->vhost_requires_shm_log
dev->vhost_ops->vhost_set_backend_cap
dev->vhost_ops->vhost_set_vring_busyloop_timeout
dev->vhost_ops->vhost_vq_get_addr
hdev->vhost_ops->vhost_dev_start
hdev->vhost_ops->vhost_get_config
hdev->vhost_ops->vhost_get_inflight_fd
hdev->vhost_ops->vhost_net_set_backend
hdev->vhost_ops->vhost_set_config
hdev->vhost_ops->vhost_set_inflight_fd
hdev->vhost_ops->vhost_set_iotlb_callback

So we should Implement all of the vhost_ops callbacks, forwarding them
to actual vhost_backed, and delete conditionally these ones? In other
words, dynamically generate the new shadow vq vhost_ops? If a new
callback is added to any vhost backend in the future, do we have to
force the adding / checking for NULL in shadow backend vhost_ops?
Would this be a good moment to check if all backends implement these
and delete the checks?

I think it won't be easy if we want to support all kinds of vhost 
backends from the start. So we can go with vhost-vdpa one first.

Actually how it work might be something like (no need to switch 
vhost_ops, we can do everything silently in the ops)

1) when device to switch to shadow vq (e.g via QMP)
2) vhost-vdpa will stop and sync state (last_avail_idx) internally
3) reset vhost-vdpa, clean call and kick eventfd
4) allocate vqs for vhost-vdpa, new call and kick eventfd, restart 
vhost-vdpa
5) start the shadow vq (make it start for last_avail_idx)
6) intercept ioeventfd and forward the request to callfd
7) intercept callfd and forward the request to irqfd
8) forward request between shadow virtqueue and vhost-vdpa

There are also checks like:

if (dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER)

How would shadow_vq backend expose itself? (I guess as the actual used backend).

I can modify this patchset to not relay the guest->host notifications
on vq handlers but on eventfd handlers. Although this will make it
independent of the actual virtio device kind used, I can see two
drawbacks:
* The actual fact that it makes it independent of virtio device kind.
If a device does not use the notifiers and poll the ring by itself, it
has no chance of knowing that it should stop. What happens if
virtio-net tx timer is armed when we start shadow vq?.

So if we do that in vhost level, it's a vhost backend from the virtio 
layer. Then we don't need to worry about tx timer stuffs.

* The fixes (current and future) in vq notifications, like the one
currently implemented in virtio_notify_irqfd for windows drivers
regarding ISR bit 0. I think this one in particular is OK not to
carry, but I think many changes affecting any of the functions will
have to be mirrored in the other.

Consider we behave like a vhost, it just work as in the past for other 
type of vhost backends when MSI-X is not enabled?

Thanks

Thoughts on this?

Thanks!

quoted

Thanks

quoted

+static uint16_t vhost_shadow_vring_used_flags(VhostShadowVirtqueue *svq)
+{
+    const struct vring_used *used = svq->hvq->used;
+    return virtio_tswap16(svq->vdev, used->flags);
+}
+
+static bool vhost_shadow_vring_should_kick(VhostShadowVirtqueue *vq)
+{
+    return !(vhost_shadow_vring_used_flags(vq) & VRING_USED_F_NO_NOTIFY);
+}
+
+static void vhost_shadow_vring_kick(VhostShadowVirtqueue *vq)
+{
+    if (vhost_shadow_vring_should_kick(vq)) {
+        event_notifier_set(&vq->kick_notifier);
+    }
+}
+
+static void handle_shadow_vq(VirtIODevice *vdev, VirtQueue *vq)
+{
+    struct vhost_dev *hdev = vhost_dev_from_virtio(vdev);
+    uint16_t idx = virtio_get_queue_index(vq);
+
+    VhostShadowVirtqueue *svq = hdev->shadow_vqs[idx];
+
+    vhost_shadow_vring_kick(svq);
+}
+
+/*
+ * Start shadow virtqueue operation.
+ * @dev vhost device
+ * @svq Shadow Virtqueue
+ *
+ * Run in RCU context
+ */
+bool vhost_shadow_vq_start_rcu(struct vhost_dev *dev,
+                               VhostShadowVirtqueue *svq)
+{
+    const VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(dev->vdev);
+    EventNotifier *vq_host_notifier = virtio_queue_get_host_notifier(svq->vq);
+    unsigned idx = virtio_queue_get_idx(svq->vdev, svq->vq);
+    struct vhost_vring_file kick_file = {
+        .index = idx,
+        .fd = event_notifier_get_fd(&svq->kick_notifier),
+    };
+    int r;
+    bool ok;
+
+    /* Check that notifications are still going directly to vhost dev */
+    assert(virtio_queue_host_notifier_status(svq->vq));
+
+    ok = k->set_vq_handler(dev->vdev, idx, handle_shadow_vq);
+    if (!ok) {
+        error_report("Couldn't set the vq handler");
+        goto err_set_kick_handler;
+    }
+
+    r = dev->vhost_ops->vhost_set_vring_kick(dev, &kick_file);
+    if (r != 0) {
+        error_report("Couldn't set kick fd: %s", strerror(errno));
+        goto err_set_vring_kick;
+    }
+
+    event_notifier_set_handler(vq_host_notifier,
+                               virtio_queue_host_notifier_read);
+    virtio_queue_set_host_notifier_enabled(svq->vq, false);
+    virtio_queue_host_notifier_read(vq_host_notifier);
+
+    return true;
+
+err_set_vring_kick:
+    k->set_vq_handler(dev->vdev, idx, NULL);
+
+err_set_kick_handler:
+    return false;
+}
+
+/*
+ * Stop shadow virtqueue operation.
+ * @dev vhost device
+ * @svq Shadow Virtqueue
+ *
+ * Run in RCU context
+ */
+void vhost_shadow_vq_stop_rcu(struct vhost_dev *dev,
+                              VhostShadowVirtqueue *svq)
+{
+    const VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(svq->vdev);
+    unsigned idx = virtio_queue_get_idx(svq->vdev, svq->vq);
+    EventNotifier *vq_host_notifier = virtio_queue_get_host_notifier(svq->vq);
+    struct vhost_vring_file kick_file = {
+        .index = idx,
+        .fd = event_notifier_get_fd(vq_host_notifier),
+    };
+    int r;
+
+    /* Restore vhost kick */
+    r = dev->vhost_ops->vhost_set_vring_kick(dev, &kick_file);
+    /* Cannot do a lot of things */
+    assert(r == 0);
+
+    event_notifier_set_handler(vq_host_notifier, NULL);
+    virtio_queue_set_host_notifier_enabled(svq->vq, true);
+    k->set_vq_handler(svq->vdev, idx, NULL);
+}
+
   /*
    * Creates vhost shadow virtqueue, and instruct vhost device to use the shadow
    * methods and file descriptors.

@@ -24,8 +138,13 @@ typedef struct VhostShadowVirtqueue {
   VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev *dev, int idx)
   {
       g_autofree VhostShadowVirtqueue *svq = g_new0(VhostShadowVirtqueue, 1);
+    int vq_idx = dev->vhost_ops->vhost_get_vq_index(dev, dev->vq_index + idx);
       int r;

+    svq->vq = virtio_get_queue(dev->vdev, vq_idx);
+    svq->hvq = &dev->vqs[idx];
+    svq->vdev = dev->vdev;
+
       r = event_notifier_init(&svq->kick_notifier, 0);
       if (r != 0) {
           error_report("Couldn't create kick event notifier: %s",

@@ -40,7 +159,7 @@ VhostShadowVirtqueue *vhost_shadow_vq_new(struct vhost_dev *dev, int idx)
           goto err_init_call_notifier;
       }

-    return svq;
+    return g_steal_pointer(&svq);

   err_init_call_notifier:
       event_notifier_cleanup(&svq->kick_notifier);

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 42836e45f3..bde688f278 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c

@@ -25,6 +25,7 @@
   #include "exec/address-spaces.h"
   #include "hw/virtio/virtio-bus.h"
   #include "hw/virtio/virtio-access.h"
+#include "hw/virtio/vhost-shadow-virtqueue.h"
   #include "migration/blocker.h"
   #include "migration/qemu-file-types.h"
   #include "sysemu/dma.h"

@@ -945,6 +946,82 @@ static void vhost_log_global_stop(MemoryListener *listener)
       }
   }

+static int vhost_sw_live_migration_stop(struct vhost_dev *dev)
+{
+    int idx;
+
+    WITH_RCU_READ_LOCK_GUARD() {
+        dev->sw_lm_enabled = false;
+
+        for (idx = 0; idx < dev->nvqs; ++idx) {
+            vhost_shadow_vq_stop_rcu(dev, dev->shadow_vqs[idx]);
+        }
+    }
+
+    for (idx = 0; idx < dev->nvqs; ++idx) {
+        vhost_shadow_vq_free(dev->shadow_vqs[idx]);
+    }
+
+    g_free(dev->shadow_vqs);
+    dev->shadow_vqs = NULL;
+    return 0;
+}
+
+static int vhost_sw_live_migration_start(struct vhost_dev *dev)
+{
+    int idx;
+
+    dev->shadow_vqs = g_new0(VhostShadowVirtqueue *, dev->nvqs);
+    for (idx = 0; idx < dev->nvqs; ++idx) {
+        dev->shadow_vqs[idx] = vhost_shadow_vq_new(dev, idx);
+        if (unlikely(dev->shadow_vqs[idx] == NULL)) {
+            goto err;
+        }
+    }
+
+    WITH_RCU_READ_LOCK_GUARD() {
+        for (idx = 0; idx < dev->nvqs; ++idx) {
+            int stop_idx = idx;
+            bool ok = vhost_shadow_vq_start_rcu(dev,
+                                                dev->shadow_vqs[idx]);
+
+            if (!ok) {
+                while (--stop_idx >= 0) {
+                    vhost_shadow_vq_stop_rcu(dev, dev->shadow_vqs[stop_idx]);
+                }
+
+                goto err;
+            }
+        }
+    }
+
+    dev->sw_lm_enabled = true;
+    return 0;
+
+err:
+    for (; idx >= 0; --idx) {
+        vhost_shadow_vq_free(dev->shadow_vqs[idx]);
+    }
+    g_free(dev->shadow_vqs[idx]);
+
+    return -1;
+}
+
+static int vhost_sw_live_migration_enable(struct vhost_dev *dev,
+                                          bool enable_lm)
+{
+    int r;
+
+    if (enable_lm == dev->sw_lm_enabled) {
+        return 0;
+    }
+
+    r = enable_lm ? vhost_sw_live_migration_start(dev)
+                  : vhost_sw_live_migration_stop(dev);
+
+    return r;
+}
+
   static void vhost_log_start(MemoryListener *listener,
                               MemoryRegionSection *section,
                               int old, int new)

@@ -1389,6 +1466,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
       hdev->log = NULL;
       hdev->log_size = 0;
       hdev->log_enabled = false;
+    hdev->sw_lm_enabled = false;
       hdev->started = false;
       memory_listener_register(&hdev->memory_listener, &address_space_memory);
       QLIST_INSERT_HEAD(&vhost_devices, hdev, entry);

@@ -1816,6 +1894,11 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
           hdev->vhost_ops->vhost_dev_start(hdev, false);
       }
       for (i = 0; i < hdev->nvqs; ++i) {
+        if (hdev->sw_lm_enabled) {
+            vhost_shadow_vq_stop_rcu(hdev, hdev->shadow_vqs[i]);
+            vhost_shadow_vq_free(hdev->shadow_vqs[i]);
+        }
+
           vhost_virtqueue_stop(hdev,
                                vdev,
                                hdev->vqs + i,

@@ -1829,6 +1912,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
           memory_listener_unregister(&hdev->iommu_listener);
       }
       vhost_log_put(hdev, true);
+    g_free(hdev->shadow_vqs);
+    hdev->sw_lm_enabled = false;
       hdev->started = false;
       hdev->vdev = NULL;
   }

@@ -1845,5 +1930,53 @@ int vhost_net_set_backend(struct vhost_dev *hdev,

   void qmp_x_vhost_enable_shadow_vq(const char *name, bool enable, Error **errp)
   {
-    error_setg(errp, "Shadow virtqueue still not implemented.");
+    struct vhost_dev *hdev;
+    const char *err_cause = NULL;
+    const VirtioDeviceClass *k;
+    int r;
+    ErrorClass err_class = ERROR_CLASS_GENERIC_ERROR;
+
+    QLIST_FOREACH(hdev, &vhost_devices, entry) {
+        if (hdev->vdev && 0 == strcmp(hdev->vdev->name, name)) {
+            break;
+        }
+    }
+
+    if (!hdev) {
+        err_class = ERROR_CLASS_DEVICE_NOT_FOUND;
+        err_cause = "Device not found";
+        goto err;
+    }
+
+    if (!hdev->started) {
+        err_cause = "Device is not started";
+        goto err;
+    }
+
+    if (hdev->acked_features & BIT_ULL(VIRTIO_F_RING_PACKED)) {
+        err_cause = "Use packed vq";
+        goto err;
+    }
+
+    if (vhost_dev_has_iommu(hdev)) {
+        err_cause = "Device use IOMMU";
+        goto err;
+    }
+
+    k = VIRTIO_DEVICE_GET_CLASS(hdev->vdev);
+    if (!k->set_vq_handler) {
+        err_cause = "Virtio device type does not support reset of vq handler";
+        goto err;
+    }
+
+    r = vhost_sw_live_migration_enable(hdev, enable);
+    if (unlikely(r)) {
+        err_cause = "Error enabling (see monitor)";
+    }
+
+err:
+    if (err_cause) {
+        error_set(errp, err_class,
+                  "Can't enable shadow vq on %s: %s", name, err_cause);
+    }
   }

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