Re: [PATCH v5 2/6] vhost-user: add GET_VRING_BASE_SKIP_DRAIN message

On 7/29/26 14:14, Michael S. Tsirkin wrote:
> On Tue, Jul 28, 2026 at 03:08:37PM +0500, Alexandr Moshkov wrote:
>> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT was introduced to allow
>> the back-end to suspend in-flight I/O during GET_VRING_BASE instead of
>> draining it, enabling live migration of in-flight requests. However,
>> this behaviour is tied to the protocol feature itself — once negotiated,
> Can we please avoid em-dashes and all kind of unicode fanciness in
> commit log? I worry if we do this the next step is emojis) Let's stick
> to ASCII with the exception of UTF-8 in contributor names.
>
> Thanks.
Sorry,Ithinkit's a problemwiththe translatorI sometimesuse.
>
>> there is no way for the front-end to tell the back-end to fall back to
>> the normal drain behaviour on a per-stop basis. This makes it impossible
>> to selectively use suspend semantics only during live migration while
>> keeping drain semantics in other cases.
>
> So what are the "other cases"?

Sorry, I think was misleading. other cases  for example is vm 
stop/pause, reconnect. In this scenarios backend will perform drain, 
same as current qemu version.

>
>> Introduce a separate message VHOST_USER_GET_VRING_BASE_SKIP_DRAIN (id=45)
>> guarded by a new protocol feature
>> VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN. The message is identical
>> to GET_VRING_BASE except that the back-end must immediately suspend all
>> in-flight I/O and record it in the inflight region. This way the front-end
>> has explicit per-call control: send GET_VRING_BASE for normal drain, send
>> GET_VRING_BASE_SKIP_DRAIN when immediate suspend is needed.
>>
>> The new feature requires both VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT
>> and VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD to be negotiated.
>>
>> Signed-off-by: Alexandr Moshkov<[email protected]>
>> ---
>>   docs/interop/vhost-user.rst       | 88 +++++++++++++++++++------------
>>   hw/virtio/vhost-user.c            | 44 ++++++++++++++--
>>   hw/virtio/vhost.c                 |  2 +-
>>   include/hw/virtio/vhost-backend.h |  1 +
>>   include/hw/virtio/vhost-user.h    |  1 +
>>   5 files changed, 95 insertions(+), 41 deletions(-)
>>
>> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
>> index c83ae2accb..d2e0efaeaa 100644
>> --- a/docs/interop/vhost-user.rst
>> +++ b/docs/interop/vhost-user.rst
>> @@ -445,6 +445,7 @@ replies, except for the following requests:
>>   * ``VHOST_USER_GET_FEATURES``
>>   * ``VHOST_USER_GET_PROTOCOL_FEATURES``
>>   * ``VHOST_USER_GET_VRING_BASE``
>> +* ``VHOST_USER_GET_VRING_BASE_SKIP_DRAIN``
>>   * ``VHOST_USER_SET_LOG_BASE`` (if ``VHOST_USER_PROTOCOL_F_LOG_SHMFD``)
>>   * ``VHOST_USER_GET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
>>   
>> @@ -523,7 +524,7 @@ must start a ring upon receiving a kick (that is, detecting that file
>>   descriptor is readable) on the descriptor specified by
>>   ``VHOST_USER_SET_VRING_KICK`` or receiving the in-band message
>>   ``VHOST_USER_VRING_KICK`` if negotiated, and stop a ring upon receiving
>> -``VHOST_USER_GET_VRING_BASE``.
>> +``VHOST_USER_GET_VRING_BASE`` or ``VHOST_USER_GET_VRING_BASE_SKIP_DRAIN``.
>>   
>>   Rings can be enabled or disabled by ``VHOST_USER_SET_VRING_ENABLE``.
>>   
>> @@ -1101,29 +1102,30 @@ 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_GET_VRING_BASE_INFLIGHT 20
>> -  #define VHOST_USER_PROTOCOL_F_GPA_ADDRESSES           21
>> -  #define VHOST_USER_PROTOCOL_F_SHMEM_MAP               22
>> +  #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
>> +  #define VHOST_USER_PROTOCOL_F_GPA_ADDRESSES             21
>> +  #define VHOST_USER_PROTOCOL_F_SHMEM_MAP                 22
>> +  #define VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN 23
>>   
>>   Front-end message types
>>   -----------------------
>> @@ -1320,17 +1322,11 @@ Front-end message types
>>     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 suspend in-flight I/O
>> -  requests and record them as described in :ref:`Inflight I/O tracking
>> -  <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 negotiated if
>> -  ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` is also negotiated.
>> +  specified vring before stopping it. If the
>> +  ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN`` protocol feature has
>> +  been negotiated, the front-end may instead use
>> +  ``VHOST_USER_GET_VRING_BASE_SKIP_DRAIN`` to request the back-end to
>> +  suspend in-flight I/O immediately.
>>   
>>   ``VHOST_USER_SET_VRING_KICK``
>>     :id: 12
>> @@ -1833,6 +1829,28 @@ Front-end message types
>>   
>>     * The size may be 0 if the region is unused.
>>   
>> +``VHOST_USER_GET_VRING_BASE_SKIP_DRAIN``
>> +  :id: 45
>> +  :equivalent ioctl: N/A
>> +  :request payload: vring state description
>> +  :reply payload: vring descriptor index/indices
>> +
>> +  This message requires the ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN``
>> +  protocol feature to be negotiated.
>> +
>> +  Identical to ``VHOST_USER_GET_VRING_BASE`` except that the back-end
>> +  must not wait for inflight I/O requests to complete before stopping
>> +  the vring.  Instead, the back-end must immediately suspend all
>> +  in-flight I/O requests and record them as described in
>> +  :ref:`Inflight I/O tracking <inflight_io_tracking>`. 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_SKIP_DRAIN`` protocol feature
>> +  must only be negotiated if both ``VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT``
>> +  and ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD`` are also negotiated.
>> +
>>   Back-end message types
>>   ----------------------
>>   
>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>> index d627351f45..bf455daa44 100644
>> --- a/hw/virtio/vhost-user.c
>> +++ b/hw/virtio/vhost-user.c
>> @@ -101,6 +101,7 @@ typedef enum VhostUserRequest {
>>       VHOST_USER_SET_DEVICE_STATE_FD = 42,
>>       VHOST_USER_CHECK_DEVICE_STATE = 43,
>>       VHOST_USER_GET_SHMEM_CONFIG = 44,
>> +    VHOST_USER_GET_VRING_BASE_SKIP_DRAIN = 45,
>>       VHOST_USER_MAX
>>   } VhostUserRequest;
>>   
>> @@ -167,6 +168,7 @@ static const char *vhost_req_name(VhostUserRequest req)
>>       VHOST_USER_CASE(GET_SHARED_OBJECT)
>>       VHOST_USER_CASE(SET_DEVICE_STATE_FD)
>>       VHOST_USER_CASE(CHECK_DEVICE_STATE)
>> +    VHOST_USER_CASE(GET_VRING_BASE_SKIP_DRAIN)
>>       default:
>>           return "<unknown>";
>>       }
>> @@ -1399,12 +1401,18 @@ static VhostUserHostNotifier *fetch_notifier(VhostUserState *u,
>>       return g_ptr_array_index(u->notifiers, idx);
>>   }
>>   
>> -static int vhost_user_get_vring_base(struct vhost_dev *dev,
>> -                                     struct vhost_vring_state *ring)
>> +static int get_vring_base(struct vhost_dev *dev,
>> +                          struct vhost_vring_state *ring,
>> +                          bool skip_drain)
>>   {
>>       int ret;
>> +    int request = VHOST_USER_GET_VRING_BASE;
>> +    if (skip_drain) {
>> +        request = VHOST_USER_GET_VRING_BASE_SKIP_DRAIN;
>> +    }
>> +
>>       VhostUserMsg msg = {
>> -        .hdr.request = VHOST_USER_GET_VRING_BASE,
>> +        .hdr.request = request,
>>           .hdr.flags = VHOST_USER_VERSION,
>>           .payload.state = *ring,
>>           .hdr.size = sizeof(msg.payload.state),
>> @@ -1424,9 +1432,9 @@ static int vhost_user_get_vring_base(struct vhost_dev *dev,
>>           return ret;
>>       }
>>   
>> -    if (msg.hdr.request != VHOST_USER_GET_VRING_BASE) {
>> +    if (msg.hdr.request != request) {
>>           error_report("Received unexpected msg type. Expected %d received %d",
>> -                     VHOST_USER_GET_VRING_BASE, msg.hdr.request);
>> +                     request, msg.hdr.request);
>>           return -EPROTO;
>>       }
>>   
>> @@ -1440,6 +1448,25 @@ static int vhost_user_get_vring_base(struct vhost_dev *dev,
>>       return 0;
>>   }
>>   
>> +static int vhost_user_get_vring_base(struct vhost_dev *dev,
>> +                                     struct vhost_vring_state *ring)
>> +{
>> +    return get_vring_base(dev, ring, false);
>> +}
>> +
>> +static int vhost_user_get_vring_base_skip_drain(struct vhost_dev *dev,
>> +                                     struct vhost_vring_state *ring)
>> +{
>> +    bool skip_drain_supported = vhost_user_has_protocol_feature(dev,
>> +                               VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN);
>> +
>> +    if (!skip_drain_supported) {
>> +        return 0;
>> +    }
>> +
>> +    return get_vring_base(dev, ring, true);
>> +}
>> +
>>   static int vhost_set_vring_file(struct vhost_dev *dev,
>>                                   VhostUserRequest request,
>>                                   struct vhost_vring_file *file)
>> @@ -2551,6 +2578,12 @@ static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
>>                                  VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT);
>>           }
>>   
>> +        if (!virtio_has_feature(protocol_features,
>> +                                VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT)) {
>> +            protocol_features &= ~(1ULL <<
>> +                             VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN);
>> +        }
>> +
>>           /* final set of protocol features */
>>           u->protocol_features = protocol_features;
>>           err = vhost_user_set_protocol_features(dev, u->protocol_features);
>> @@ -3409,6 +3442,7 @@ const VhostOps user_ops = {
>>           .vhost_set_vring_num = vhost_user_set_vring_num,
>>           .vhost_set_vring_base = vhost_user_set_vring_base,
>>           .vhost_get_vring_base = vhost_user_get_vring_base,
>> +        .vhost_get_vring_base_skip_drain = vhost_user_get_vring_base_skip_drain,
>>           .vhost_set_vring_kick = vhost_user_set_vring_kick,
>>           .vhost_set_vring_call = vhost_user_set_vring_call,
>>           .vhost_set_vring_err = vhost_user_set_vring_err,
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index 2a62550222..2c7f464d55 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -1502,7 +1502,7 @@ static int do_vhost_virtqueue_stop(struct vhost_dev *dev,
>>   {
>>       if (skip_drain) {
>>           assert(vhost_user_has_protocol_feature(dev,
>> -               VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT));
>> +               VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN));
>>       }
>>       int vhost_vq_index = dev->vhost_ops->vhost_get_vq_index(dev, idx);
>>       struct vhost_vring_state state = {
>> diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h
>> index d878d7b733..daa979a1aa 100644
>> --- a/include/hw/virtio/vhost-backend.h
>> +++ b/include/hw/virtio/vhost-backend.h
>> @@ -190,6 +190,7 @@ typedef struct VhostOps {
>>       vhost_set_vring_num_op vhost_set_vring_num;
>>       vhost_set_vring_base_op vhost_set_vring_base;
>>       vhost_get_vring_base_op vhost_get_vring_base;
>> +    vhost_get_vring_base_op vhost_get_vring_base_skip_drain;
>>       vhost_set_vring_kick_op vhost_set_vring_kick;
>>       vhost_set_vring_call_op vhost_set_vring_call;
>>       vhost_set_vring_err_op vhost_set_vring_err;
>> diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
>> index 06c360af18..47c13f8677 100644
>> --- a/include/hw/virtio/vhost-user.h
>> +++ b/include/hw/virtio/vhost-user.h
>> @@ -36,6 +36,7 @@ enum VhostUserProtocolFeature {
>>       VHOST_USER_PROTOCOL_F_GET_VRING_BASE_INFLIGHT = 20,
>>       VHOST_USER_PROTOCOL_F_GPA_ADDRESSES = 21,
>>       VHOST_USER_PROTOCOL_F_SHMEM = 22,
>> +    VHOST_USER_PROTOCOL_F_GET_VRING_BASE_SKIP_DRAIN = 23,
>>       VHOST_USER_PROTOCOL_F_MAX
>>   };
>>   
>> -- 
>> 2.34.1