Re: [PATCH v5 2/5] vhost-user: introduce protocol feature for skip drain on GET_VRING_BASE

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jan 12, 2026 at 04:45:00PM +0500, Alexandr Moshkov wrote:
> Add vhost-user protocol feature
> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT
> 
> Now on GET_VRING_BASE this feature can control whether to wait for
> in-flight requests to complete or not.
> 
> It will be helpfull in future for in-flight requests migration in
> vhost-user devices.
> 
> Signed-off-by: Alexandr Moshkov <dtalexundeer@yandex-team.ru>
> ---
>  docs/interop/vhost-user.rst    | 52 ++++++++++++++++++----------------
>  include/hw/virtio/vhost-user.h |  1 +
>  2 files changed, 29 insertions(+), 24 deletions(-)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 02908b48fa..80c80aada5 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -1033,26 +1033,27 @@ Protocol features
>  
>  .. code:: c
>  
> -  #define VHOST_USER_PROTOCOL_F_MQ                    0
> -  #define VHOST_USER_PROTOCOL_F_LOG_SHMFD             1
> -  #define VHOST_USER_PROTOCOL_F_RARP                  2
> -  #define VHOST_USER_PROTOCOL_F_REPLY_ACK             3
> -  #define VHOST_USER_PROTOCOL_F_MTU                   4
> -  #define VHOST_USER_PROTOCOL_F_BACKEND_REQ           5
> -  #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN          6
> -  #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION        7
> -  #define VHOST_USER_PROTOCOL_F_PAGEFAULT             8
> -  #define VHOST_USER_PROTOCOL_F_CONFIG                9
> -  #define VHOST_USER_PROTOCOL_F_BACKEND_SEND_FD      10
> -  #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER        11
> -  #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD       12
> -  #define VHOST_USER_PROTOCOL_F_RESET_DEVICE         13
> -  #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14
> -  #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS  15
> -  #define VHOST_USER_PROTOCOL_F_STATUS               16
> -  #define VHOST_USER_PROTOCOL_F_XEN_MMAP             17
> -  #define VHOST_USER_PROTOCOL_F_SHARED_OBJECT        18
> -  #define VHOST_USER_PROTOCOL_F_DEVICE_STATE         19
> +  #define VHOST_USER_PROTOCOL_F_MQ                       0
> +  #define VHOST_USER_PROTOCOL_F_LOG_SHMFD                1
> +  #define VHOST_USER_PROTOCOL_F_RARP                     2
> +  #define VHOST_USER_PROTOCOL_F_REPLY_ACK                3
> +  #define VHOST_USER_PROTOCOL_F_MTU                      4
> +  #define VHOST_USER_PROTOCOL_F_BACKEND_REQ              5
> +  #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN             6
> +  #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION           7
> +  #define VHOST_USER_PROTOCOL_F_PAGEFAULT                8
> +  #define VHOST_USER_PROTOCOL_F_CONFIG                   9
> +  #define VHOST_USER_PROTOCOL_F_BACKEND_SEND_FD         10
> +  #define VHOST_USER_PROTOCOL_F_HOST_NOTIFIER           11
> +  #define VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD          12
> +  #define VHOST_USER_PROTOCOL_F_RESET_DEVICE            13
> +  #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS    14
> +  #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS     15
> +  #define VHOST_USER_PROTOCOL_F_STATUS                  16
> +  #define VHOST_USER_PROTOCOL_F_XEN_MMAP                17
> +  #define VHOST_USER_PROTOCOL_F_SHARED_OBJECT           18
> +  #define VHOST_USER_PROTOCOL_F_DEVICE_STATE            19
> +  #define VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT 20
>  
>  Front-end message types
>  -----------------------
> @@ -1243,11 +1244,14 @@ Front-end message types
>  
>    When and as long as all of a device's vrings are stopped, it is
>    *suspended*, see :ref:`Suspended device state
> -  <suspended_device_state>`. The back-end must complete all inflight I/O
> -  requests for the specified vring before stopping it.
> +  <suspended_device_state>`.
>  
> -  The request payload's *num* field is currently reserved and must be
> -  set to 0.
> +  By default, the back-end must complete all inflight I/O requests for the
> +  specified vring before stopping it.
> +
> +  If the ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT`` protocol feature
> +  has been negotiated, the back-end may stop the vring immediately without
> +  waiting for inflight I/O requests to complete.

This paragraph is not specific enough. It gives the impression that I/O
requests can be left running, but that's not the case. They need to be
quiesced and recorded in the Inflight I/O Tracking
(VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD) shared memory data structure.

I suggest rewording it as follows:

  If the ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT`` protocol
  feature has been negotiated, the back-end may suspend in-flight I/O
  requests and record them as described in :ref:`inflight-io-tracking`
  instead of completing them before stopping the vring. How to suspend
  an in-flight request depends on the implementation of the back-end but
  it typically can be done by aborting or cancelling the underlying I/O
  request. The ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT``
  protocol feature must only be neogotiated if
  ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` is also negotiated.

(A _inflight-io-tracking label needs to be added in order to reference
the "Inflight I/O tracking" section.)

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