From: "Michael S. Tsirkin"<mst@redhat.com>
Date:  Mon, Mar 2, 2026, 08:45
Subject:  Re: [BUG] vhost_net: livelock in handle_rx() when GRO packet exceeds virtqueue capacity
To: "ShuangYu"<redacted>
Cc: "jasowang@redhat.com"<jasowang@redhat.com>, "virtualization@lists.linux.dev"<redacted>, "netdev@vger.kernel.org"<redacted>, "linux-kernel@vger.kernel.org"<redacted>, "kvm@vger.kernel.org"<redacted>
On Sun, Mar 01, 2026 at 07:39:30PM -0500, Michael S. Tsirkin wrote:

quoted

On Sun, Mar 01, 2026 at 07:10:06PM -0500, Michael S. Tsirkin wrote:

quoted

On Sun, Mar 01, 2026 at 10:36:39PM +0000, ShuangYu wrote:

quoted

Hi,

We have hit a severe livelock in vhost_net on 6.18.x. The vhost
kernel thread spins at 100% CPU indefinitely in handle_rx(), and
QEMU becomes unkillable (stuck in D state).
[This is a text/plain messages]

Environment
-----------
  Kernel:  6.18.10-1.el8.elrepo.x86_64
  QEMU:    7.2.19
  Virtio:  VIRTIO_F_IN_ORDER is negotiated
  Backend: vhost (kernel)

Symptoms
--------
  - vhost-<pid> kernel thread at 100% CPU (R state, never yields)
  - QEMU stuck in D state at vhost_dev_flush() after receiving SIGTERM
  - kill -9 has no effect on the QEMU process
  - libvirt management plane deadlocks ("cannot acquire state change lock")

Root Cause
----------
The livelock is triggered when a GRO-merged packet on the host TAP
interface (e.g., ~60KB) exceeds the remaining free capacity of the
guest's RX virtqueue (e.g., ~40KB of available buffers).

The loop in handle_rx() (drivers/vhost/net.c) proceeds as follows:

  1. get_rx_bufs() calls vhost_get_vq_desc_n() to fetch descriptors.
    It advances vq->last_avail_idx and vq->next_avail_head as it
    consumes buffers, but runs out before satisfying datalen.

  2. get_rx_bufs() jumps to err: and calls
    vhost_discard_vq_desc(vq, headcount, n), which rolls back
    vq->last_avail_idx and vq->next_avail_head.

    Critically, vq->avail_idx (the cached copy of the guest's
    avail->idx) is NOT rolled back. This is correct behavior in
    isolation, but creates a persistent mismatch:

      vq->avail_idx      = 108  (cached, unchanged)
      vq->last_avail_idx = 104  (rolled back)

  3. handle_rx() sees headcount == 0 and calls vhost_enable_notify().
    Inside, vhost_get_avail_idx() finds:

      vq->avail_idx (108) != vq->last_avail_idx (104)

    It returns 1 (true), indicating "new buffers available."
    But these are the SAME buffers that were just discarded.

  4. handle_rx() hits `continue`, restarting the loop.

  5. In the next iteration, vhost_get_vq_desc_n() checks:

      if (vq->avail_idx == vq->last_avail_idx)

    This is FALSE (108 != 104), so it skips re-reading the guest's
    actual avail->idx and directly fetches the same descriptors.

  6. The exact same sequence repeats: fetch -> too small -> discard
    -> rollback -> "new buffers!" -> continue. Indefinitely.

This appears to be a regression introduced by the VIRTIO_F_IN_ORDER
support, which added vhost_get_vq_desc_n() with the cached avail_idx
short-circuit check, and the two-argument vhost_discard_vq_desc()
with next_avail_head rollback. The mismatch between the rollback
scope (last_avail_idx, next_avail_head) and the check scope
(avail_idx vs last_avail_idx) was not present before this change.

bpftrace Evidence
-----------------
During the 100% CPU lockup, we traced:

  @get_rx_ret[0]:      4468052   // get_rx_bufs() returns 0 every time
  @peek_ret[60366]:    4385533   // same 60KB packet seen every iteration
  @sock_err[recvmsg]:        0   // tun_recvmsg() is never reached

vhost_get_vq_desc_n() was observed iterating over the exact same 11
descriptor addresses millions of times per second.

Workaround
----------
Either of the following avoids the livelock:

  - Disable GRO/GSO on the TAP interface:
     ethtool -K <tap> gro off gso off

  - Switch from kernel vhost to userspace QEMU backend:
     <driver name='qemu'/> in libvirt XML

Bisect
------
We have not yet completed a full git bisect, but the issue does not
occur on 6.17.x kernels which lack the VIRTIO_F_IN_ORDER vhost
support. We will follow up with a Fixes: tag if we can identify the
exact commit.

Suggested Fix Direction
-----------------------
In handle_rx(), when get_rx_bufs() returns 0 (headcount == 0) due to
insufficient buffers (not because the queue is truly empty), the code
should break out of the loop rather than relying on
vhost_enable_notify() to make that determination. For example, when
get_rx_bufs() returns r == 0 with datalen still > 0, this indicates a
"packet too large" condition, not a "queue empty" condition, and
should be handled differently.

Thanks,
ShuangYu

Hmm. On a hunch, does the following help? completely untested,
it is night here, sorry.

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 2f2c45d20883..aafae15d5156 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c

@@ -1522,6 +1522,7 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)

 static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
 {
         __virtio16 idx;
+        u16 avail_idx;
         int r;
 
         r = vhost_get_avail(vq, idx, &vq->avail->idx);

@@ -1532,17 +1533,19 @@ static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)

         }
 
         /* Check it isn't doing very strange thing with available indexes */
-        vq->avail_idx = vhost16_to_cpu(vq, idx);
-        if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
+        avail_idx = vhost16_to_cpu(vq, idx);
+        if (unlikely((u16)(avail_idx - vq->last_avail_idx) > vq->num)) {
                 vq_err(vq, "Invalid available index change from %u to %u",
                        vq->last_avail_idx, vq->avail_idx);
                 return -EINVAL;
         }
 
         /* We're done if there is nothing new */
-        if (vq->avail_idx == vq->last_avail_idx)
+        if (avail_idx == vq->avail_idx)
                 return 0;
 
+        vq->avail_idx == avail_idx;
+

meaning 
        vq->avail_idx = avail_idx; 
of course

quoted

         /*
          * We updated vq->avail_idx so we need a memory barrier between
          * the index read above and the caller reading avail ring entries.

and the change this is fixing was done in d3bb267bbdcba199568f1325743d9d501dea0560

-- 
MST

Thank you for the quick fix and for identifying the root commit.

I've reviewed the patch and I believe the logic is correct — changing
the "nothing new" check in vhost_get_avail_idx() from comparing
against vq->last_avail_idx to comparing against the cached
vq->avail_idx makes it immune to the rollback done by
vhost_discard_vq_desc(), which is exactly what breaks the loop.

One minor nit: the vq_err message on the sanity check path still
references vq->avail_idx before it has been updated:

        vq_err(vq, "Invalid available index change from %u to %u",
-              vq->last_avail_idx, vq->avail_idx);
+              vq->last_avail_idx, avail_idx);


Since this issue was found in production, I need some time to prepare a test setup to verify the patch

Thanks,
ShuangYu