Re: [RFC v5 3/7] vhost: Forward descriptors to device via packed SVQ

[Sahil Siddiq <icegambit91@gmail.com>]

Hi,

I had a few queries here.

On 3/24/25 7:29 PM, Sahil Siddiq wrote:
> Implement the insertion of available buffers in the descriptor area of
> packed shadow virtqueues. It takes into account descriptor chains, but
> does not consider indirect descriptors.
> 
> Enable the packed SVQ to forward the descriptors to the device.
> 
> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>
> ---
> Changes from v4 -> v5:
> - This was commit #2 in v4. This has been reordered to commit #3
>    based on review comments.
> - vhost-shadow-virtqueue.c:
>    (vhost_svq_valid_features): Move addition of enums to commit #6
>    based on review comments.
>    (vhost_svq_add_packed): Set head_idx to buffer id instead of vring's
>    index.
>    (vhost_svq_kick): Split into vhost_svq_kick_split and
>    vhost_svq_kick_packed.
>    (vhost_svq_add): Use new vhost_svq_kick_* functions.
> 
>   hw/virtio/vhost-shadow-virtqueue.c | 117 +++++++++++++++++++++++++++--
>   1 file changed, 112 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
> index 4f74ad402a..6e16cd4bdf 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.c
> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> @@ -193,10 +193,83 @@ static void vhost_svq_add_split(VhostShadowVirtqueue *svq,
>       /* Update the avail index after write the descriptor */
>       smp_wmb();
>       avail->idx = cpu_to_le16(svq->shadow_avail_idx);
> +}
> +
> +/**
> + * Write descriptors to SVQ packed vring
> + *
> + * @svq: The shadow virtqueue
> + * @out_sg: The iovec to the guest
> + * @out_num: Outgoing iovec length
> + * @in_sg: The iovec from the guest
> + * @in_num: Incoming iovec length
> + * @sgs: Cache for hwaddr
> + * @head: Saves current free_head
> + */
> +static void vhost_svq_add_packed(VhostShadowVirtqueue *svq,
> +                                 const struct iovec *out_sg, size_t out_num,
> +                                 const struct iovec *in_sg, size_t in_num,
> +                                 hwaddr *sgs, unsigned *head)
> +{
> +    uint16_t id, curr, i, head_flags = 0, head_idx;
> +    size_t num = out_num + in_num;
> +    unsigned n;
> +
> +    struct vring_packed_desc *descs = svq->vring_packed.vring.desc;
> +
> +    head_idx = svq->vring_packed.next_avail_idx;

Since "svq->vring_packed.next_avail_idx" is part of QEMU internals and not
stored in guest memory, no endianness conversion is required here, right?

> +    i = head_idx;
> +    id = svq->free_head;
> +    curr = id;
> +    *head = id;

Should head be the buffer id or the idx of the descriptor ring where the
first descriptor of a descriptor chain is inserted?

> +    /* Write descriptors to SVQ packed vring */
> +    for (n = 0; n < num; n++) {
> +        uint16_t flags = cpu_to_le16(svq->vring_packed.avail_used_flags |
> +                                     (n < out_num ? 0 : VRING_DESC_F_WRITE) |
> +                                     (n + 1 == num ? 0 : VRING_DESC_F_NEXT));
> +        if (i == head_idx) {
> +            head_flags = flags;
> +        } else {
> +            descs[i].flags = flags;
> +        }
> +
> +        descs[i].addr = cpu_to_le64(sgs[n]);
> +        descs[i].id = id;
> +        if (n < out_num) {
> +            descs[i].len = cpu_to_le32(out_sg[n].iov_len);
> +        } else {
> +            descs[i].len = cpu_to_le32(in_sg[n - out_num].iov_len);
> +        }
> +
> +        curr = cpu_to_le16(svq->desc_next[curr]);
> +
> +        if (++i >= svq->vring_packed.vring.num) {
> +            i = 0;
> +            svq->vring_packed.avail_used_flags ^=
> +                1 << VRING_PACKED_DESC_F_AVAIL |
> +                1 << VRING_PACKED_DESC_F_USED;
> +        }
> +    }
>   
> +    if (i <= head_idx) {
> +        svq->vring_packed.avail_wrap_counter ^= 1;
> +    }
> +
> +    svq->vring_packed.next_avail_idx = i;
> +    svq->shadow_avail_idx = i;
> +    svq->free_head = curr;
> +
> +    /*
> +     * A driver MUST NOT make the first descriptor in the list
> +     * available before all subsequent descriptors comprising
> +     * the list are made available.
> +     */
> +    smp_wmb();
> +    svq->vring_packed.vring.desc[head_idx].flags = head_flags;
>   }
>   
> -static void vhost_svq_kick(VhostShadowVirtqueue *svq)
> +static void vhost_svq_kick_split(VhostShadowVirtqueue *svq)
>   {
>       bool needs_kick;
>   
> @@ -209,7 +282,8 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq)
>       if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
>           uint16_t avail_event = le16_to_cpu(
>                   *(uint16_t *)(&svq->vring.used->ring[svq->vring.num]));
> -        needs_kick = vring_need_event(avail_event, svq->shadow_avail_idx, svq->shadow_avail_idx - 1);
> +        needs_kick = vring_need_event(avail_event, svq->shadow_avail_idx,
> +                     svq->shadow_avail_idx - 1);
>       } else {
>           needs_kick =
>                   !(svq->vring.used->flags & cpu_to_le16(VRING_USED_F_NO_NOTIFY));
> @@ -222,6 +296,30 @@ static void vhost_svq_kick(VhostShadowVirtqueue *svq)
>       event_notifier_set(&svq->hdev_kick);
>   }
>   
> +static void vhost_svq_kick_packed(VhostShadowVirtqueue *svq)
> +{
> +    bool needs_kick;
> +
> +    /*
> +     * We need to expose the available array entries before checking
> +     * notification suppressions.
> +     */
> +    smp_mb();
> +
> +    if (virtio_vdev_has_feature(svq->vdev, VIRTIO_RING_F_EVENT_IDX)) {
> +        return;
> +    } else {
> +        needs_kick = (svq->vring_packed.vring.device->flags !=
> +                      cpu_to_le16(VRING_PACKED_EVENT_FLAG_DISABLE));
> +    }
> +
> +    if (!needs_kick) {
> +        return;
> +    }
> +
> +    event_notifier_set(&svq->hdev_kick);
> +}
> +
>   /**
>    * Add an element to a SVQ.
>    *
> @@ -258,13 +356,22 @@ int vhost_svq_add(VhostShadowVirtqueue *svq, const struct iovec *out_sg,
>           return -EINVAL;
>       }
>   
> -    vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> -                        in_num, sgs, &qemu_head);
> +    if (svq->is_packed) {
> +        vhost_svq_add_packed(svq, out_sg, out_num, in_sg,
> +                             in_num, sgs, &qemu_head);
> +    } else {
> +        vhost_svq_add_split(svq, out_sg, out_num, in_sg,
> +                            in_num, sgs, &qemu_head);
> +    }
>   
>       svq->num_free -= ndescs;
>       svq->desc_state[qemu_head].elem = elem;
>       svq->desc_state[qemu_head].ndescs = ndescs;

*head in vhost_svq_add_packed() is stored in "qemu_head" here.

> -    vhost_svq_kick(svq);
> +    if (svq->is_packed) {
> +        vhost_svq_kick_packed(svq);
> +    } else {
> +        vhost_svq_kick_split(svq);
> +    }
>       return 0;
>   }
>   

Thanks,
Sahil