Re: [PATCH v1 13/13] virtio/vhost-user: dynamically assign VhostUserHostNotifiers

[Stefan Hajnoczi]

On Mon, 21 Mar 2022 at 11:59, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> At a couple of hundred bytes per notifier allocating one for every
> potential queue is very wasteful as most devices only have a few
> queues. Instead of having this handled statically dynamically assign
> them and track in a GPtrArray.
>
> [AJB: it's hard to trigger the vhost notifiers code, I assume as it
> requires a KVM guest with appropriate backend]

I think vhost works with TCG. There is ioeventfd emulation in QEMU's
memory dispatch code, see memory_region_dispatch_write_eventfds().
There is irqfd emulation code for VIRTIO devices in
virtio_queue_set_guest_notifier_fd_handler().

Why do you say it's hard to trigger?

Stefan

>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  include/hw/virtio/vhost-user.h | 42 ++++++++++++++++-
>  hw/virtio/vhost-user.c         | 83 +++++++++++++++++++++++++++-------
>  hw/virtio/trace-events         |  1 +
>  3 files changed, 108 insertions(+), 18 deletions(-)
>
> diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
> index 6e0e8a71a3..c6e693cd3f 100644
> --- a/include/hw/virtio/vhost-user.h
> +++ b/include/hw/virtio/vhost-user.h
> @@ -11,21 +11,61 @@
>  #include "chardev/char-fe.h"
>  #include "hw/virtio/virtio.h"
>
> +/**
> + * VhostUserHostNotifier - notifier information for one queue
> + * @rcu: rcu_head for cleanup
> + * @mr: memory region of notifier
> + * @addr: current mapped address
> + * @unmap_addr: address to be un-mapped
> + * @idx: virtioqueue index
> + *
> + * The VhostUserHostNotifier entries are re-used. When an old mapping
> + * is to be released it is moved to @unmap_addr and @addr is replaced.
> + * Once the RCU process has completed the unmap @unmap_addr is
> + * cleared.
> + */
>  typedef struct VhostUserHostNotifier {
>      struct rcu_head rcu;
>      MemoryRegion mr;
>      void *addr;
>      void *unmap_addr;
> +    int idx;
>  } VhostUserHostNotifier;
>
> +/**
> + * VhostUserState - shared state for all vhost-user devices
> + * @chr: the character backend for the socket
> + * @notifiers: GPtrArray of @VhostUserHostnotifier
> + * @memory_slots:
> + */
>  typedef struct VhostUserState {
>      CharBackend *chr;
> -    VhostUserHostNotifier notifier[VIRTIO_QUEUE_MAX];
> +    GPtrArray *notifiers;
>      int memory_slots;
>      bool supports_config;
>  } VhostUserState;
>
> +/**
> + * vhost_user_init() - initialise shared vhost_user state
> + * @user: allocated area for storing shared state
> + * @chr: the chardev for the vhost socket
> + * @errp: error handle
> + *
> + * User can either directly g_new() space for the state or embed
> + * VhostUserState in their larger device structure and just point to
> + * it.
> + *
> + * Return: true on success, false on error while setting errp.
> + */
>  bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp);
> +
> +/**
> + * vhost_user_cleanup() - cleanup state
> + * @user: ptr to use state
> + *
> + * Cleans up shared state and notifiers, callee is responsible for
> + * freeing the @VhostUserState memory itself.
> + */
>  void vhost_user_cleanup(VhostUserState *user);
>
>  #endif
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 6ce082861b..4c0423de55 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1174,14 +1174,16 @@ static void 
> vhost_user_host_notifier_free(VhostUserHostNotifier *n)
>      n->unmap_addr = NULL;
>  }
>
> -static void vhost_user_host_notifier_remove(VhostUserState *user,
> -                                            VirtIODevice *vdev, int 
> queue_idx)
> +/*
> + * clean-up function for notifier, will finally free the structure
> + * under rcu.
> + */
> +static void vhost_user_host_notifier_remove(VhostUserHostNotifier *n,
> +                                            VirtIODevice *vdev)
>  {
> -    VhostUserHostNotifier *n = &user->notifier[queue_idx];
> -
>      if (n->addr) {
>          if (vdev) {
> -            virtio_queue_set_host_notifier_mr(vdev, queue_idx, &n->mr, 
> false);
> +            virtio_queue_set_host_notifier_mr(vdev, n->idx, &n->mr, false);
>          }
>          assert(!n->unmap_addr);
>          n->unmap_addr = n->addr;
> @@ -1225,6 +1227,15 @@ static int vhost_user_set_vring_enable(struct 
> vhost_dev *dev, int enable)
>      return 0;
>  }
>
> +static VhostUserHostNotifier *fetch_notifier(VhostUserState *u,
> +                                             int idx)
> +{
> +    if (idx >= u->notifiers->len) {
> +        return NULL;
> +    }
> +    return g_ptr_array_index(u->notifiers, idx);
> +}
> +
>  static int vhost_user_get_vring_base(struct vhost_dev *dev,
>                                       struct vhost_vring_state *ring)
>  {
> @@ -1237,7 +1248,10 @@ static int vhost_user_get_vring_base(struct vhost_dev 
> *dev,
>      };
>      struct vhost_user *u = dev->opaque;
>
> -    vhost_user_host_notifier_remove(u->user, dev->vdev, ring->index);
> +    VhostUserHostNotifier *n = fetch_notifier(u->user, ring->index);
> +    if (n) {
> +        vhost_user_host_notifier_remove(n, dev->vdev);
> +    }
>
>      ret = vhost_user_write(dev, &msg, NULL, 0);
>      if (ret < 0) {
> @@ -1502,6 +1516,29 @@ static int 
> vhost_user_slave_handle_config_change(struct vhost_dev *dev)
>      return dev->config_ops->vhost_dev_config_notifier(dev);
>  }
>
> +/*
> + * Fetch or create the notifier for a given idx. Newly created
> + * notifiers are added to the pointer array that tracks them.
> + */
> +static VhostUserHostNotifier *fetch_or_create_notifier(VhostUserState *u,
> +                                                       int idx)
> +{
> +    VhostUserHostNotifier *n = NULL;
> +    if (idx >= u->notifiers->len) {
> +        g_ptr_array_set_size(u->notifiers, idx);
> +    }
> +
> +    n = g_ptr_array_index(u->notifiers, idx);
> +    if (!n) {
> +        n = g_new0(VhostUserHostNotifier, 1);
> +        n->idx = idx;
> +        g_ptr_array_insert(u->notifiers, idx, n);
> +        trace_vhost_user_create_notifier(idx, n);
> +    }
> +
> +    return n;
> +}
> +
>  static int vhost_user_slave_handle_vring_host_notifier(struct vhost_dev *dev,
>                                                         VhostUserVringArea 
> *area,
>                                                         int fd)
> @@ -1521,9 +1558,12 @@ static int 
> vhost_user_slave_handle_vring_host_notifier(struct vhost_dev *dev,
>          return -EINVAL;
>      }
>
> -    n = &user->notifier[queue_idx];
> -
> -    vhost_user_host_notifier_remove(user, vdev, queue_idx);
> +    /*
> +     * Fetch notifier and invalidate any old data before setting up
> +     * new mapped address.
> +     */
> +    n = fetch_or_create_notifier(user, queue_idx);
> +    vhost_user_host_notifier_remove(n, vdev);
>
>      if (area->u64 & VHOST_USER_VRING_NOFD_MASK) {
>          return 0;
> @@ -2526,6 +2566,20 @@ static int vhost_user_set_inflight_fd(struct vhost_dev 
> *dev,
>      return vhost_user_write(dev, &msg, &inflight->fd, 1);
>  }
>
> +static void vhost_user_state_destroy(gpointer data)
> +{
> +    VhostUserHostNotifier *n = (VhostUserHostNotifier *) data;
> +    if (n) {
> +        vhost_user_host_notifier_remove(n, NULL);
> +        object_unparent(OBJECT(&n->mr));
> +        /*
> +         * We can't free until vhost_user_host_notifier_remove has
> +         * done it's thing so schedule the free with RCU.
> +         */
> +        g_free_rcu(n, rcu);
> +    }
> +}
> +
>  bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp)
>  {
>      if (user->chr) {
> @@ -2534,23 +2588,18 @@ bool vhost_user_init(VhostUserState *user, 
> CharBackend *chr, Error **errp)
>      }
>      user->chr = chr;
>      user->memory_slots = 0;
> +    user->notifiers = g_ptr_array_new_full(VIRTIO_QUEUE_MAX / 4,
> +                                           &vhost_user_state_destroy);
>      return true;
>  }
>
>  void vhost_user_cleanup(VhostUserState *user)
>  {
> -    int i;
> -    VhostUserHostNotifier *n;
> -
>      if (!user->chr) {
>          return;
>      }
>      memory_region_transaction_begin();
> -    for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
> -        n = &user->notifier[i];
> -        vhost_user_host_notifier_remove(user, NULL, i);
> -        object_unparent(OBJECT(&n->mr));
> -    }
> +    user->notifiers = (GPtrArray *) g_ptr_array_free(user->notifiers, true);
>      memory_region_transaction_commit();
>      user->chr = NULL;
>  }
> diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> index fd213e2a27..b40392a593 100644
> --- a/hw/virtio/trace-events
> +++ b/hw/virtio/trace-events
> @@ -23,6 +23,7 @@ vhost_user_postcopy_waker_found(uint64_t client_addr) 
> "0x%"PRIx64
>  vhost_user_postcopy_waker_nomatch(const char *rb, uint64_t rb_offset) "%s + 
> 0x%"PRIx64
>  vhost_user_read(uint32_t req, uint32_t flags) "req:%d flags:0x%"PRIx32""
>  vhost_user_write(uint32_t req, uint32_t flags) "req:%d flags:0x%"PRIx32""
> +vhost_user_create_notifier(int idx, void *n) "idx:%d n:%p"
>
>  # vhost-vdpa.c
>  vhost_vdpa_dma_map(void *vdpa, int fd, uint32_t msg_type, uint64_t iova, 
> uint64_t size, uint64_t uaddr, uint8_t perm, uint8_t type) "vdpa:%p fd: %d 
> msg_type: %"PRIu32" iova: 0x%"PRIx64" size: 0x%"PRIx64" uaddr: 0x%"PRIx64" 
> perm: 0x%"PRIx8" type: %"PRIu8
> --
> 2.30.2
>
>