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

[Jason Wang <jasowang@redhat.com>]

在 2021/3/17 下午10:38, Eugenio Perez Martin 写道:
> On Wed, Mar 17, 2021 at 3:51 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/3/17 上午12:05, Eugenio Perez Martin 写道:
>>> On Tue, Mar 16, 2021 at 9:15 AM Jason Wang <jasowang@redhat.com> wrote:
>>>> 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.
>>>>
>>> I was trying to expose the buffer with the same endianness as the
>>> driver originally offered, so if guest->qemu requires a bswap, I think
>>> it will always require a bswap again to expose to the device again.
>>
>> So assumes vhost-vDPA always provide a non-transitional device[1].
>>
>> Then if Qemu present a transitional device, we need to do the endian
>> conversion when necessary, if Qemu present a non-transitional device, we
>> don't need to do that, guest driver will do that for us.
>>
>> But it looks to me the virtio_tswap() can't be used for this since it:
>>
>> static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
>> {
>> #if defined(LEGACY_VIRTIO_IS_BIENDIAN)
>>       return virtio_is_big_endian(vdev);
>> #elif defined(TARGET_WORDS_BIGENDIAN)
>>       if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
>>           /* Devices conforming to VIRTIO 1.0 or later are always LE. */
>>           return false;
>>       }
>>       return true;
>> #else
>>       return false;
>> #endif
>> }
>>
>> So if we present a legacy device on top of a non-transitiaonl vDPA
>> device. The VIRITIO_F_VERSION_1 check is wrong.
>>
>>
>>>> For vhost-vDPA, we can assume that it's a 1.0 device.
>>> Isn't it needed if the host is big endian?
>>
>> [1]
>>
>> So non-transitional device always assume little endian.
>>
>> For vhost-vDPA, we don't want to present transitional device which may
>> end up with a lot of burdens.
>>
>> I suspect the legacy driver plust vhost vDPA already break, so I plan to
>> mandate VERSION_1 for all vDPA devices.
>>
> Right. I think it's the best then.
>
> However, then we will need a similar method to always expose
> address/length as little endian, isn't it?


Yes.


>
>>>>> +        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?
>>>>
>>> This code follows the pattern of virtio_blk_handle_vq: we jump out of
>>> the inner while, and we re-enable the notifications. After that, we
>>> check for updates on guest avail_idx.
>>
>> Ok, but this will end up with a lot of unnecessary kicks without event
>> index.
>>
> I can move the kick out of the inner loop, but that could add latency.


So I think the right way is to disable the notification until some 
buffers are consumed by used ring.


>
>>>>> +            }
>>>>> +
>>>>> +            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?
>>>>
>>> Do you mean pending as in-flight/processing in the device? The device
>>> must be paused at this point.
>>
>> Ok. I see there's a vhost_set_vring_enable(dev, false) in
>> vhost_sw_live_migration_start().
>>
>>
>>> Currently there is no assertion for
>>> this, maybe we can track the device status for it.
>>>
>>> For the queue handlers to be running at this point, the main event
>>> loop should serialize QMP and handlers as far as I know (and they
>>> would make all state inconsistent if the device stops suddenly). It
>>> would need to be synchronized if the handlers run in their own AIO
>>> context. That would be nice to have but it's not included here.
>>
>> That's why I suggest to just drop the QMP stuffs and use cli parameters
>> to enable shadow virtqueue. Things would be greatly simplified I guess.
>>
> I can send a series without it, but SVQ will need to be able to kick
> in dynamically sooner or later if we want to use it for live
> migration.


I'm not sure I get the issue here. My understnading is everyhing will be 
processed in the same aio context.

Thanks


>
>> Thanks
>>
>>
>>>> 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;
>>>>>             }
>>>>>