On Sun, Mar 01, 2026 at 10:36:39PM +0000, ShuangYu wrote:
> 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;
+
/*
* We updated vq->avail_idx so we need a memory barrier between
* the index read above and the caller reading avail ring entries.
