Incorrect feature negotiation for vhost-vdpa netdevice

[Gautam Dawar]

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);
}

[Gautam Dawar (gdawar)]

QEMU version: 5.1.0

[Gautam Dawar (gdawar)]

https://qemu-devel.nongnu.narkive.com/jUimpLt0/patch-vhost-net-initialize-acked-features-to-a-safe-value-during-ack
This review of a patch that introduced "acked_features = backend_features" behaviour suggests that acked_features should be 0 by default, but it ended up pushing it as it is now (as they say that not setting acked_features to backend_features sometimes makes acked_features value 'unexpected')

Other devices initialize acked_features to guest_features (basically what VM writes to driver_features ANDed with device_features), changing default value to any of those (0 as review initially suggested, or guest_features) should fix this problem for us

[Thomas Huth (th-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.

[Thomas Huth (th-huth)]

This ticket has been moved here (thanks, Gautam):
https://gitlab.com/qemu-project/qemu/-/issues/331
... thus I'm closing this on Launchpad now.

[Gautam Dawar (gdawar)]

Thanks Thomas Huth.

I couldn't find an option to assign the issue on gitlab to anyone. Can you please help with that?

[Thomas Huth (th-huth)]

Not sure who should be the assignee here ... maybe it's best if you write a mail to the people who have been involved in the original code (see https://gitlab.com/qemu-project/qemu/-/commit/108a64818e69be0a97c ) and ask them who could have a look at this issue.