Re: [PATCH v4] qapi: net: deprecate vhostforce option

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> This option for tap and vhost-user netdevs doesn't make sense
> since long ago (10 years!), starting from commits:
>
>  1e7398a140f7a6 ("vhost: enable vhost without without MSI-X")
>  24f938a682d934 ("vhost user:support vhost user nic for non msi guests")
>
> Prior these commits, to enable kernel vhost-net, or vhost-user-net for
> some specific kind of guests (that don't have MSI-X support), you should
> have set vhostforce=on.
>
> Now guest type doesn't matter, all guests are equal for these
> options logic.
>
> For tap the current logic is:
>   vhost=on / vhost=off : vhostforce ignored, doesn't make sense
>   vhost unset : vhostforce counts, enabling vhost
>
> So you may enable vhost for tap several ways:
> - vhost=on
> - vhostforce=on
> - vhost=on + vhostforce=on
> - and even vhost=on + vhostforce=off
>
> - they are all equal.
>
> For vhost-user we simply ignore the vhostforce option at all in the
> code.
>
> Let's finally deprecate the extra options.
>
> Also, fix @vhostforce documentation everywhere to show the real picture,
> and update vhost-user test to not use deprecated option.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
>
> v4:
> - update documentation
> - deprecate also similar option for vhost-user net
>
> v3 was "[PATCH v3] qapi: net/tap: deprecate vhostforce option":
> Supersedes: <20250901153943.65235-1-vsementsov@yandex-team.ru>
>
>  docs/about/deprecated.rst     | 11 +++++++++++
>  qapi/net.json                 | 20 ++++++++++++++++----
>  qemu-options.hx               | 13 +++++--------
>  tests/qtest/vhost-user-test.c |  2 +-
>  4 files changed, 33 insertions(+), 13 deletions(-)
>
> diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> index 98361f5832..2882603b20 100644
> --- a/docs/about/deprecated.rst
> +++ b/docs/about/deprecated.rst
> @@ -487,6 +487,17 @@ Stream ``reconnect`` (since 9.2)
>  The ``reconnect`` option only allows specifying second granularity timeouts,
>  which is not enough for all types of use cases, use ``reconnect-ms`` instead.
>  
> +TAP ``vhostforce`` (since 10.2)
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The ``vhostforce`` option is redundant with the ``vhost`` option.
> +If they conflict, ``vhost`` takes precedence.  Just use ``vhost``.
> +
> +Vhost-user ``vhostforce`` (since 10.2)
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +The ``vhostforce`` option is a no-op.  Do not use it.
> +
>  CPU device properties
>  '''''''''''''''''''''
>  
> diff --git a/qapi/net.json b/qapi/net.json
> index 60d196afe5..531b360e46 100644
> --- a/qapi/net.json
> +++ b/qapi/net.json
> @@ -346,13 +346,20 @@
>  # @vhostfds: file descriptors of multiple already opened vhost net
>  #     devices
>  #
> -# @vhostforce: vhost on for non-MSIX virtio guests
> +# @vhostforce: enable vhost-net network accelerator.  Ignored when
> +#    @vhost is set.
>  #
>  # @queues: number of queues to be created for multiqueue capable tap
>  #
>  # @poll-us: maximum number of microseconds that could be spent on busy
>  #     polling for tap (since 2.7)
>  #
> +# Features:
> +#
> +# @deprecated: Member @vhostforce is deprecated.  The @vhostforce
> +#    option is redundant with the @vhost option. If they conflict,
> +#    @vhost takes precedence.  Just use @vhost.

Behavior should be discussed in the descriptions instead.  I think what
we already have there ("Ignored when @vhost is set") suffices.

Let's use the common phrasing here:

   # @deprecated: Member @vhostforce is deprecated.  Use @vhost instead.

> +#
>  # Since: 1.2
>  ##
>  { 'struct': 'NetdevTapOptions',
> @@ -369,7 +376,7 @@
>      '*vhost':      'bool',
>      '*vhostfd':    'str',
>      '*vhostfds':   'str',
> -    '*vhostforce': 'bool',
> +    '*vhostforce': { 'type': 'bool', 'features': [ 'deprecated' ] },
>      '*queues':     'uint32',
>      '*poll-us':    'uint32'} }
>  
> @@ -606,17 +613,22 @@
>  #
>  # @chardev: name of a unix socket chardev
>  #
> -# @vhostforce: vhost on for non-MSIX virtio guests (default: false).
> +# @vhostforce: no-op (default: false).

Suggest

   # @vhostforce: silently ignored, exists just for backward
   #     compatibility.

>  #
>  # @queues: number of queues to be created for multiqueue vhost-user
>  #     (default: 1) (Since 2.5)
>  #
> +# Features:
> +#
> +# @deprecated: Member @vhostforce is deprecated.  The @vhostforce
> +#    option is ignored in code and does nothing.  Don't use it.

Again, discussion of behavior does not belong here.  I think

   # @deprecated: Member @vhostforce is deprecated.

suffices.

> +#
>  # Since: 2.1
>  ##
>  { 'struct': 'NetdevVhostUserOptions',
>    'data': {
>      'chardev':        'str',
> -    '*vhostforce':    'bool',
> +    '*vhostforce': { 'type': 'bool', 'features': [ 'deprecated' ] },
>      '*queues':        'int' } }
>  
>  ##
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 0223ceffeb..35a70096e8 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -2882,7 +2882,7 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
>  #else
>      "-netdev tap,id=str[,fd=h][,fds=x:y:...:z][,ifname=name][,script=file][,downscript=dfile]\n"
>      "         [,br=bridge][,helper=helper][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off]\n"
> -    "         [,vhostfd=h][,vhostfds=x:y:...:z][,vhostforce=on|off][,queues=n]\n"
> +    "         [,vhostfd=h][,vhostfds=x:y:...:z][,queues=n]\n"
>      "         [,poll-us=n]\n"
>      "                configure a host TAP network backend with ID 'str'\n"
>      "                connected to a bridge (default=" DEFAULT_BRIDGE_INTERFACE ")\n"
> @@ -2898,9 +2898,7 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
>      "                default is disabled 'sndbuf=0' to enable flow control set 'sndbuf=1048576')\n"
>      "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
>      "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
> -    "                use vhost=on to enable experimental in kernel accelerator\n"
> -    "                    (only has effect for virtio guests which use MSIX)\n"
> -    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
> +    "                use vhost=on to enable in kernel accelerator\n"

So the kernel's accelerator is no longer experimental?

>      "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
>      "                use 'vhostfds=x:y:...:z to connect to multiple already opened vhost net devices\n"
>      "                use 'queues=n' to specify the number of queues to be created for multiqueue TAP\n"
> @@ -2991,7 +2989,7 @@ DEF("netdev", HAS_ARG, QEMU_OPTION_netdev,
>      "                use 'start-queue=m' to specify the first queue that should be used\n"
>  #endif
>  #ifdef CONFIG_POSIX
> -    "-netdev vhost-user,id=str,chardev=dev[,vhostforce=on|off]\n"
> +    "-netdev vhost-user,id=str,chardev=dev\n"
>      "                configure a vhost-user network, backed by a chardev 'dev'\n"
>  #endif
>  #ifdef __linux__
> @@ -3882,12 +3880,11 @@ SRST
>      for insertion into the socket map.  The combination of 'map-path' and
>      'sock-fds' together is not supported.
>  
> -``-netdev vhost-user,chardev=id[,vhostforce=on|off][,queues=n]``
> +``-netdev vhost-user,chardev=id[,queues=n]``
>      Establish a vhost-user netdev, backed by a chardev id. The chardev
>      should be a unix domain socket backed one. The vhost-user uses a
>      specifically defined protocol to pass vhost ioctl replacement
> -    messages to an application on the other end of the socket. On
> -    non-MSIX guests, the feature can be forced with vhostforce. Use
> +    messages to an application on the other end of the socket. Use
>      'queues=n' to specify the number of queues to be created for
>      multiqueue vhost-user.
>  
> diff --git a/tests/qtest/vhost-user-test.c b/tests/qtest/vhost-user-test.c
> index 609ff24059..a4862d7087 100644
> --- a/tests/qtest/vhost-user-test.c
> +++ b/tests/qtest/vhost-user-test.c
> @@ -46,7 +46,7 @@
>  #define QEMU_CMD_SHM    " -m %d -object memory-backend-shm,id=mem,size=%dM," \
>                          " -numa node,memdev=mem"
>  #define QEMU_CMD_CHR    " -chardev socket,id=%s,path=%s%s"
> -#define QEMU_CMD_NETDEV " -netdev vhost-user,id=hs0,chardev=%s,vhostforce=on"
> +#define QEMU_CMD_NETDEV " -netdev vhost-user,id=hs0,chardev=%s"
>  
>  #define HUGETLBFS_MAGIC       0x958458f6