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

[Raphael Norwitz]

On Mon, Sep 28, 2020 at 9:17 AM 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.
>
> --
> ChangeList:
> v3:
> -make used_memslots a member of struct vhost_dev instead of a global static 
> value
>
> v2:
> -eliminating useless used_memslots_exceeded variable and 
> used_memslots_is_exceeded() API
>
> v1:
> -vhost-user: add separate memslot counter for vhost-user
>
> Signed-off-by: Jiajun Chen <chenjiajun8@huawei.com>
> Signed-off-by: Jianjay Zhou <jianjay.zhou@huawei.com>

I'm happy with this from a vhost/vhost-user perspective. vhost-backend
change looks good too. I'm a little confused by what's going on with
net/vhost-user.c.

> ---
>  hw/virtio/vhost-backend.c         | 12 ++++++++++
>  hw/virtio/vhost-user.c            | 25 +++++++++++++++++++++
>  hw/virtio/vhost.c                 | 37 +++++++++++++++++++++++--------
>  include/hw/virtio/vhost-backend.h |  5 +++++
>  include/hw/virtio/vhost.h         |  1 +
>  net/vhost-user.c                  |  7 ++++++
>  6 files changed, 78 insertions(+), 9 deletions(-)
>

> diff --git a/net/vhost-user.c b/net/vhost-user.c
> index 17532daaf3..7e93955537 100644
> --- a/net/vhost-user.c
> +++ b/net/vhost-user.c
> @@ -20,6 +20,7 @@
>  #include "qemu/error-report.h"
>  #include "qemu/option.h"
>  #include "trace.h"
> +#include "include/hw/virtio/vhost.h"
>
>  typedef struct NetVhostUserState {
>      NetClientState nc;
> @@ -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);

Can you elaborate on this check here? What does it have to do with
fixing memslots accounting? Maybe it should be in a separate change?

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