Re: [PING][PING][PATCH v2] vhost: make SET_VRING_ADDR, SET_FEATURES send replies

[Philippe Mathieu-Daudé]

Cc more ppl.

On 7/29/21 12:56 PM, Denis Plotnikov wrote:
> 
> On 23.07.2021 12:59, Denis Plotnikov wrote:
>>
>> ping!
>>
>> On 19.07.2021 17:21, Denis Plotnikov wrote:
>>> On vhost-user-blk migration, qemu normally sends a number of commands
>>> to enable logging if VHOST_USER_PROTOCOL_F_LOG_SHMFD is negotiated.
>>> Qemu sends VHOST_USER_SET_FEATURES to enable buffers logging and
>>> VHOST_USER_SET_VRING_ADDR per each started ring to enable "used ring"
>>> data logging.
>>> The issue is that qemu doesn't wait for reply from the vhost daemon
>>> for these commands which may result in races between qemu expectation
>>> of logging starting and actual login starting in vhost daemon.
>>>
>>> The race can appear as follows: on migration setup, qemu enables dirty page
>>> logging by sending VHOST_USER_SET_FEATURES. The command doesn't arrive to a
>>> vhost-user-blk daemon immediately and the daemon needs some time to turn the
>>> logging on internally. If qemu doesn't wait for reply, after sending the
>>> command, qemu may start migrate memory pages to a destination. At this time,
>>> the logging may not be actually turned on in the daemon but some guest 
>>> pages,
>>> which the daemon is about to write to, may have already been transferred
>>> without logging to the destination. Since the logging wasn't turned on,
>>> those pages won't be transferred again as dirty. So we may end up with
>>> corrupted data on the destination.
>>> The same scenario is applicable for "used ring" data logging, which is
>>> turned on with VHOST_USER_SET_VRING_ADDR command.
>>>
>>> To resolve this issue, this patch makes qemu wait for the commands result
>>> explicilty if VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated and
>>> logging is enabled.
>>>
>>> Signed-off-by: Denis Plotnikov <den-plotnikov@yandex-team.ru>
>>> ---
>>> v1 -> v2:
>>>   * send reply only when logging is enabled [mst]
>>>
>>> v0 -> v1:
>>>   * send reply for SET_VRING_ADDR, SET_FEATURES only [mst]
>>>   
>>>  hw/virtio/vhost-user.c | 37 ++++++++++++++++++++++++++++++++++---
>>>  1 file changed, 34 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
>>> index ee57abe04526..133588b3961e 100644
>>> --- a/hw/virtio/vhost-user.c
>>> +++ b/hw/virtio/vhost-user.c
>>> @@ -1095,6 +1095,11 @@ static int vhost_user_set_mem_table(struct vhost_dev 
>>> *dev,
>>>      return 0;
>>>  }
>>>  
>>> +static bool log_enabled(uint64_t features)
>>> +{
>>> +    return !!(features & (0x1ULL << VHOST_F_LOG_ALL));
>>> +}
>>> +
>>>  static int vhost_user_set_vring_addr(struct vhost_dev *dev,
>>>                                       struct vhost_vring_addr *addr)
>>>  {
>>> @@ -1105,10 +1110,21 @@ static int vhost_user_set_vring_addr(struct 
>>> vhost_dev *dev,
>>>          .hdr.size = sizeof(msg.payload.addr),
>>>      };
>>>  
>>> +    bool reply_supported = virtio_has_feature(dev->protocol_features,
>>> +                                              
>>> VHOST_USER_PROTOCOL_F_REPLY_ACK);
>>> +
>>> +    if (reply_supported && log_enabled(msg.hdr.flags)) {
>>> +        msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
>>> +    }
>>> +
>>>      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
>>>          return -1;
>>>      }
>>>  
>>> +    if (msg.hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
>>> +        return process_message_reply(dev, &msg);
>>> +    }
>>> +
>>>      return 0;
>>>  }
>>>  
>>> @@ -1288,7 +1304,8 @@ static int vhost_user_set_vring_call(struct vhost_dev 
>>> *dev,
>>>      return vhost_set_vring_file(dev, VHOST_USER_SET_VRING_CALL, file);
>>>  }
>>>  
>>> -static int vhost_user_set_u64(struct vhost_dev *dev, int request, uint64_t 
>>> u64)
>>> +static int vhost_user_set_u64(struct vhost_dev *dev, int request, uint64_t 
>>> u64,
>>> +                              bool need_reply)
>>>  {
>>>      VhostUserMsg msg = {
>>>          .hdr.request = request,
>>> @@ -1297,23 +1314,37 @@ static int vhost_user_set_u64(struct vhost_dev 
>>> *dev, int request, uint64_t u64)
>>>          .hdr.size = sizeof(msg.payload.u64),
>>>      };
>>>  
>>> +    if (need_reply) {
>>> +        bool reply_supported = virtio_has_feature(dev->protocol_features,
>>> +                                          VHOST_USER_PROTOCOL_F_REPLY_ACK);
>>> +        if (reply_supported) {
>>> +            msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
>>> +        }
>>> +    }
>>> +
>>>      if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
>>>          return -1;
>>>      }
>>>  
>>> +    if (msg.hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
>>> +        return process_message_reply(dev, &msg);
>>> +    }
>>> +
>>>      return 0;
>>>  }
>>>  
>>>  static int vhost_user_set_features(struct vhost_dev *dev,
>>>                                     uint64_t features)
>>>  {
>>> -    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features);
>>> +    return vhost_user_set_u64(dev, VHOST_USER_SET_FEATURES, features,
>>> +                              log_enabled(features));
>>>  }
>>>  
>>>  static int vhost_user_set_protocol_features(struct vhost_dev *dev,
>>>                                              uint64_t features)
>>>  {
>>> -    return vhost_user_set_u64(dev, VHOST_USER_SET_PROTOCOL_FEATURES, 
>>> features);
>>> +    return vhost_user_set_u64(dev, VHOST_USER_SET_PROTOCOL_FEATURES, 
>>> features,
>>> +                              false);
>>>  }
>>>  
>>>  static int vhost_user_get_u64(struct vhost_dev *dev, int request, uint64_t 
>>> *u64)