Re: [PATCH v3 18/23] vhost: introduce check_memslots() helper

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 15, 2025 at 05:58:02PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Reviewed-by: Raphael Norwitz <raphael.s.norwitz@gmail.com>
> Reviewed-by: Daniil Tatianin <d-tatianin@yandex-team.ru>


This needs a better commit log.
the subject makes it look like it's merely adding a helper
but it is not the case: e.g. errors are now handled
somewhat differently.

Pls document the thinking process explaining why it does not
matter. CB.


> ---
>  hw/virtio/vhost.c | 71 ++++++++++++++++++++++++++---------------------
>  1 file changed, 40 insertions(+), 31 deletions(-)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 34846edf36..56a812b06b 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -1572,12 +1572,50 @@ static int vhost_dev_get_features(struct vhost_dev *hdev,
>      return r;
>  }
>  
> +static bool check_memslots(struct vhost_dev *hdev, Error **errp)
> +{
> +    unsigned int used, reserved, limit;
> +
> +    limit = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
> +    if (limit < MEMORY_DEVICES_SAFE_MAX_MEMSLOTS &&
> +        memory_devices_memslot_auto_decision_active()) {
> +        error_setg(errp, "some memory device (like virtio-mem)"
> +            " decided how many memory slots to use based on the overall"
> +            " number of memory slots; this vhost backend would further"
> +            " restricts the overall number of memory slots");
> +        error_append_hint(errp, "Try plugging this vhost backend before"
> +            " plugging such memory devices.\n");
> +        return false;
> +    }
> +
> +    /*
> +     * The listener we registered properly setup the number of required

This comment worked in the original code but not now.

The listener here is memory_listener_register which yes happens to
be called before check_memslots but it is far from obvious.



> +     * memslots in vhost_commit().
> +     */
> +    used = hdev->mem->nregions;



> +
> +    /*
> +     * We assume that all reserved memslots actually require a real memslot
> +     * in our vhost backend. This might not be true, for example, if the
> +     * memslot would be ROM. If ever relevant, we can optimize for that --
> +     * but we'll need additional information about the reservations.
> +     */
> +    reserved = memory_devices_get_reserved_memslots();
> +    if (used + reserved > limit) {
> +        error_setg(errp, "vhost backend memory slots limit (%d) is less"
> +                   " than current number of used (%d) and reserved (%d)"
> +                   " memory slots for memory devices.", limit, used, reserved);
> +        return false;
> +    }
> +
> +    return true;
> +}
> +
>  int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
>                     VhostBackendType backend_type, uint32_t busyloop_timeout,
>                     Error **errp)
>  {
>      uint64_t features[VIRTIO_FEATURES_NU64S];
> -    unsigned int used, reserved, limit;
>      int i, r, n_initialized_vqs = 0;
>  
>      hdev->vdev = NULL;
> @@ -1603,19 +1641,6 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
>          goto fail;
>      }
>  
> -    limit = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
> -    if (limit < MEMORY_DEVICES_SAFE_MAX_MEMSLOTS &&
> -        memory_devices_memslot_auto_decision_active()) {
> -        error_setg(errp, "some memory device (like virtio-mem)"
> -            " decided how many memory slots to use based on the overall"
> -            " number of memory slots; this vhost backend would further"
> -            " restricts the overall number of memory slots");
> -        error_append_hint(errp, "Try plugging this vhost backend before"
> -            " plugging such memory devices.\n");
> -        r = -EINVAL;
> -        goto fail;
> -    }
> -
>      for (i = 0; i < hdev->nvqs; ++i, ++n_initialized_vqs) {
>          r = vhost_virtqueue_init(hdev, hdev->vqs + i, hdev->vq_index + i,
>                                   busyloop_timeout);
> @@ -1674,23 +1699,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void *opaque,
>      memory_listener_register(&hdev->memory_listener, &address_space_memory);


So it looks like with your change
this will temporarily register the listener
restricting the number of slots, then check and unregister.

Is this ever a problem?

commit log needs better documentation why.



>      QLIST_INSERT_HEAD(&vhost_devices, hdev, entry);
>  
> -    /*
> -     * The listener we registered properly setup the number of required
> -     * memslots in vhost_commit().
> -     */
> -    used = hdev->mem->nregions;
> -
> -    /*
> -     * We assume that all reserved memslots actually require a real memslot
> -     * in our vhost backend. This might not be true, for example, if the
> -     * memslot would be ROM. If ever relevant, we can optimize for that --
> -     * but we'll need additional information about the reservations.
> -     */
> -    reserved = memory_devices_get_reserved_memslots();
> -    if (used + reserved > limit) {
> -        error_setg(errp, "vhost backend memory slots limit (%d) is less"
> -                   " than current number of used (%d) and reserved (%d)"
> -                   " memory slots for memory devices.", limit, used, reserved);
> +    if (!check_memslots(hdev, errp)) {
>          r = -EINVAL;
>          goto fail;
>      }
> -- 
> 2.48.1