[Bug 1924603] Re: Incorrect feature negotiation for vhost-vdpa netdevice

[Thomas Huth]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1924603

Title:
  Incorrect feature negotiation for vhost-vdpa netdevice

Status in QEMU:
  Incomplete

Bug description:
  QEMU cmdline:
  =============
  ./x86_64-softmmu/qemu-system-x86_64 -machine accel=kvm -m 2G -hda  
/gautam/centos75_1.qcow2 -name gautam,process=gautam -enable-kvm -netdev 
vhost-vdpa,id=mynet0,vhostdev=/dev/vhost-vdpa-0 -device 
virtio-net-pci,netdev=mynet0,mac=02:AA:BB:DD:00:20,disable-modern=off,page-per-vq=on
 -cpu host --nographic

  Host OS:
  ========
  Linux kernel 5.11 running on x86 host

  Guest OS:
  ==========
  CentOS 7.5

  Root cause analysis:
  =====================

  For vhost-vdpa netdevice, the feature negotiation results in sending
  the superset of features received from device in call to get_features
  vdpa ops callback.

  During the feature-negotiation phase, the acknowledged feature bits
  are initialized with backend_features  and then checked for supported
  feature bits in vhost_ack_features():

  void vhost_net_ack_features(struct vhost_net *net, uint64_t features)
  {
    net->dev.acked_features = net->dev.backend_features;
    vhost_ack_features(&net->dev, vhost_net_get_feature_bits(net), features);
  }

   
  The vhost_ack_features() function just builds up on the dev.acked_features 
and never trims it down:

  void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits, 
uint64_t features)
  {     const int *bit = feature_bits;

        while (*bit != VHOST_INVALID_FEATURE_BIT) {
             uint64_t bit_mask = (1ULL << *bit);      

              if (features & bit_mask)
                   hdev->acked_features |= bit_mask;

              bit++;
         }
  }

  Because of this hdev->acked_features is always minimally equal to the
  value of device features and this is the value that is passed to the
  device in set_features callback:

  static int vhost_dev_set_features(struct vhost_dev *dev, bool enable_log)
  {
         uint64_t *features = dev->acked_features;
         .....
         r = dev->vhost_ops->*vhost_set_features*(dev, features);
  }

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