Re: [RFC v5 2/7] vhost: Data structure changes to support packed vqs

[Eugenio Perez Martin <eperezma@redhat.com>]

On Mon, Mar 24, 2025 at 3:00 PM Sahil Siddiq <icegambit91@gmail.com> wrote:
>
> Introduce "struct vring_packed".
>
> Modify VhostShadowVirtqueue so it can support split and packed virtqueue
> formats.
>
> Signed-off-by: Sahil Siddiq <sahilcdq@proton.me>
> ---
> Changes from v4 -> v5:
> - This was commit #3 in v4. This has been reordered to commit #2
>   based on review comments.
> - Place shadow_avail_idx, shadow_used_idx, last_used_idx
>   above the "shadow vring" union.
>

What is the reason for the member reorder?

>  hw/virtio/vhost-shadow-virtqueue.h | 87 +++++++++++++++++++-----------
>  1 file changed, 56 insertions(+), 31 deletions(-)
>
> diff --git a/hw/virtio/vhost-shadow-virtqueue.h b/hw/virtio/vhost-shadow-virtqueue.h
> index 9c273739d6..5f7699da9d 100644
> --- a/hw/virtio/vhost-shadow-virtqueue.h
> +++ b/hw/virtio/vhost-shadow-virtqueue.h
> @@ -46,10 +46,65 @@ typedef struct VhostShadowVirtqueueOps {
>      VirtQueueAvailCallback avail_handler;
>  } VhostShadowVirtqueueOps;
>
> +struct vring_packed {
> +    /* Actual memory layout for this queue. */
> +    struct {
> +        unsigned int num;
> +        struct vring_packed_desc *desc;
> +        struct vring_packed_desc_event *driver;
> +        struct vring_packed_desc_event *device;
> +    } vring;
> +
> +    /* Avail used flags. */
> +    uint16_t avail_used_flags;
> +
> +    /* Index of the next avail descriptor. */
> +    uint16_t next_avail_idx;
> +
> +    /* Driver ring wrap counter */
> +    bool avail_wrap_counter;
> +};
> +
>  /* Shadow virtqueue to relay notifications */
>  typedef struct VhostShadowVirtqueue {
> +    /* True if packed virtqueue */
> +    bool is_packed;
> +
> +    /* Virtio queue shadowing */
> +    VirtQueue *vq;
> +
> +    /* Virtio device */
> +    VirtIODevice *vdev;
> +
> +    /* SVQ vring descriptors state */
> +    SVQDescState *desc_state;
> +
> +    /*
> +     * Backup next field for each descriptor so we can recover securely, not
> +     * needing to trust the device access.
> +     */
> +    uint16_t *desc_next;
> +
> +    /* Next free descriptor */
> +    uint16_t free_head;
> +
> +    /* Size of SVQ vring free descriptors */
> +    uint16_t num_free;
> +
> +    /* Next head to expose to the device */
> +    uint16_t shadow_avail_idx;
> +
> +    /* Last seen used idx */
> +    uint16_t shadow_used_idx;
> +
> +    /* Next head to consume from the device */
> +    uint16_t last_used_idx;
> +
>      /* Shadow vring */
> -    struct vring vring;
> +    union {
> +        struct vring vring;
> +        struct vring_packed vring_packed;
> +    };
>
>      /* Shadow kick notifier, sent to vhost */
>      EventNotifier hdev_kick;
> @@ -69,47 +124,17 @@ typedef struct VhostShadowVirtqueue {
>      /* Guest's call notifier, where the SVQ calls guest. */
>      EventNotifier svq_call;
>
> -    /* Virtio queue shadowing */
> -    VirtQueue *vq;
> -
> -    /* Virtio device */
> -    VirtIODevice *vdev;
> -
>      /* IOVA mapping */
>      VhostIOVATree *iova_tree;
>
> -    /* SVQ vring descriptors state */
> -    SVQDescState *desc_state;
> -
>      /* Next VirtQueue element that guest made available */
>      VirtQueueElement *next_guest_avail_elem;
>
> -    /*
> -     * Backup next field for each descriptor so we can recover securely, not
> -     * needing to trust the device access.
> -     */
> -    uint16_t *desc_next;
> -
>      /* Caller callbacks */
>      const VhostShadowVirtqueueOps *ops;
>
>      /* Caller callbacks opaque */
>      void *ops_opaque;
> -
> -    /* Next head to expose to the device */
> -    uint16_t shadow_avail_idx;
> -
> -    /* Next free descriptor */
> -    uint16_t free_head;
> -
> -    /* Last seen used idx */
> -    uint16_t shadow_used_idx;
> -
> -    /* Next head to consume from the device */
> -    uint16_t last_used_idx;
> -
> -    /* Size of SVQ vring free descriptors */
> -    uint16_t num_free;
>  } VhostShadowVirtqueue;
>
>  bool vhost_svq_valid_features(uint64_t features, Error **errp);
> --
> 2.48.1
>