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

[Jason Wang <jasowang@redhat.com>]

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.


>
>>
>>> +        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.


>
>>> +            }
>>> +
>>> +            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.

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;
>>>            }
>>>
>