Re: [PATCH] vhost-user: add separate memslot counter for vhost-user

[Raphael Norwitz]

I see how this fixes vhost_has_free_slot() to correctly determine
whether or not regions can be added, but why are the
used_memslots_exceeded variable and the used_memslots_is_exceeded()
API needed?

On Mon, Aug 10, 2020 at 9:44 PM Jiajun Chen <chenjiajun8@huawei.com> wrote:
>
> Used_memslots is equal to dev->mem->nregions now, it is true for
> vhost kernel, but not for vhost user, which uses the memory regions
> that have file descriptor. In fact, not all of the memory regions
> have file descriptor.
> It is usefully in some scenarios, e.g. used_memslots is 8, and only
> 5 memory slots can be used by vhost user, it is failed to hot plug
> a new memory RAM because vhost_has_free_slot just returned false,
> but we can hot plug it safely in fact.
>
> Signed-off-by: Jiajun Chen <chenjiajun8@huawei.com>
> Signed-off-by: Jianjay Zhou <jianjay.zhou@huawei.com>
> ---
>  hw/virtio/vhost-backend.c         | 14 ++++++++
>  hw/virtio/vhost-user.c            | 28 ++++++++++++++++
>  hw/virtio/vhost.c                 | 54 +++++++++++++++++++++++++------
>  include/hw/virtio/vhost-backend.h |  5 +++
>  include/hw/virtio/vhost.h         |  1 +
>  net/vhost-user.c                  |  7 ++++
>  6 files changed, 100 insertions(+), 9 deletions(-)
>
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index 31231218dc..04d20fc3ee 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -2354,6 +2355,31 @@ void vhost_user_cleanup(VhostUserState *user)
>      user->chr = NULL;
>  }
>
> +static void vhost_user_set_used_memslots(struct vhost_dev *dev)
> +{
> +    unsigned int counter = 0;
> +    int i;
> +
> +    for (i = 0; i < dev->mem->nregions; ++i) {
> +        struct vhost_memory_region *reg = dev->mem->regions + i;
> +        ram_addr_t offset;
> +        MemoryRegion *mr;
> +
> +        assert((uintptr_t)reg->userspace_addr == reg->userspace_addr);
> +        mr = memory_region_from_host((void *)(uintptr_t)reg->userspace_addr,
> +                                    &offset);

Why not use the  vhost_user_get_mr_data helper? It would simplify the
code a bit.

> +        if (mr && memory_region_get_fd(mr) > 0) {
> +            counter++;
> +        }
> +    }
> +    vhost_user_used_memslots = counter;
> +}
> +
> +static unsigned int vhost_user_get_used_memslots(void)
> +{
> +    return vhost_user_used_memslots;
> +}
> +
>  const VhostOps user_ops = {
>          .backend_type = VHOST_BACKEND_TYPE_USER,
>          .vhost_backend_init = vhost_user_backend_init,
> @@ -2387,4 +2413,6 @@ const VhostOps user_ops = {
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 1a1384e7a6..7f36d7af25 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -1346,9 +1373,13 @@ int vhost_dev_init(struct vhost_dev *hdev, void 
> *opaque,
>      memory_listener_register(&hdev->memory_listener, &address_space_memory);
>      QLIST_INSERT_HEAD(&vhost_devices, hdev, entry);
>
> -    if (used_memslots > hdev->vhost_ops->vhost_backend_memslots_limit(hdev)) 
> {
> -        error_report("vhost backend memory slots limit is less"
> -                " than current number of present memory slots");
> +    /*
> +     * If we started a VM without any vhost device,
> +     * vhost_dev_used_memslots_is_exceeded will always return false for the
> +     * first time vhost device hot-plug(vhost_get_used_memslots is always 0),
> +     * so it needs to double check here
> +     */
> +    if (vhost_dev_used_memslots_is_exceeded(hdev)) {

Why can't we just check if hdev->vhost_ops->vhost_get_used_memslots()
> hdev->vhost_ops->vhost_backend_memslots_limit(hdev)?

>          r = -1;
>          if (busyloop_timeout) {
>              goto fail_busyloop;
> @@ -1773,3 +1804,8 @@ int vhost_net_set_backend(struct vhost_dev *hdev,
>
>      return -1;
>  }
> +
> +bool used_memslots_is_exceeded(void)
> +{
> +    return used_memslots_exceeded;
> +}
> diff --git a/net/vhost-user.c b/net/vhost-user.c
> index 17532daaf3..2f0216b518 100644
> --- a/net/vhost-user.c
> +++ b/net/vhost-user.c
> @@ -347,6 +348,12 @@ static int net_vhost_user_init(NetClientState *peer, 
> const char *device,
>          qemu_chr_fe_set_handlers(&s->chr, NULL, NULL,
>                                   net_vhost_user_event, NULL, nc0->name, NULL,
>                                   true);
> +
> +        if (used_memslots_is_exceeded()) {

Why can't you use vhost_has_free_slot() instead here?

> +            error_report("used memslots exceeded the backend limit, quit "
> +                            "loop");
> +            goto err;
> +        }
>      } while (!s->started);
>
>      assert(s->vhost_net);
> --
> 2.27.0.dirty
>
>