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

[Stefan Hajnoczi]

On Thu, Jul 29, 2021 at 02:53:53PM +0200, Philippe Mathieu-Daudé wrote:
> Cc more ppl.

This needs to go through Michael Tsirkin's tree.

Stefan

> 
> 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)
>

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