Re: [PATCH v2] net/netvsc: switch data path to synthetic on device stop

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 23 Mar 2026 16:49:57 -0700
Long Li <longli@microsoft.com> wrote:

> When DPDK stops a netvsc device (e.g. on testpmd quit), the data path
> was left pointing to the VF/MANA device. If the kernel netvsc driver
> subsequently reloads the MANA device and opens it, incoming traffic
> arrives on the MANA device immediately, before the queues are fully
> initialized. This causes bogus RX completion events to appear on the
> TX completion queue, triggering a kernel WARNING in mana_poll_tx_cq().
> 
> Fix this by switching the data path back to synthetic (via
> NVS_DATAPATH_SYNTHETIC) in hn_vf_stop() before stopping the VF device.
> This tells the host to route traffic through the synthetic path, so
> that when the MANA driver recreates its queues, no unexpected traffic
> arrives until netvsc explicitly switches back to VF.
> 
> Also update hn_vf_start() to switch the data path back to VF after the
> VF device is started, enabling correct stop/start cycling.
> 
> Both functions now use write locks instead of read locks since they
> modify vf_vsc_switched state.
> 
> Fixes: dc7680e8597c ("net/netvsc: support integrated VF")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Long Li <longli@microsoft.com>
> ---

AI spotted that ret is overwritten on error path


**Patch: [PATCH v2] net/netvsc: switch data path to synthetic on device stop**

The v2 addresses the two issues from v1 review: `hn_vf_stop()` now only clears `vf_vsc_switched` on success, and `hn_vf_start()` now stops the VF if the datapath switch fails. Both fixes are correct.

**Warning: `hn_vf_stop()` — `ret` from failed datapath switch is overwritten by `rte_eth_dev_stop()`**

When `hn_nvs_set_datapath(hv, NVS_DATAPATH_SYNTHETIC)` fails, `ret` holds that error. But execution falls through to `rte_eth_dev_stop()`, which overwrites `ret`. The caller loses the datapath switch failure — if `rte_eth_dev_stop()` succeeds, `hn_vf_stop()` returns 0 despite the datapath switch having failed. If the intent is to always stop the VF regardless of the switch result (reasonable), the datapath error should be preserved separately, or the function should return the first error. Something like:

```c
		if (hv->vf_ctx.vf_vsc_switched) {
			ret = hn_nvs_set_datapath(hv, NVS_DATAPATH_SYNTHETIC);
			if (ret) {
				PMD_DRV_LOG(ERR,
					    "Failed to switch to synthetic: %d",
					    ret);
			} else {
				hv->vf_ctx.vf_vsc_switched = false;
			}
		}

		err = rte_eth_dev_stop(vf_dev->data->port_id);
		if (err != 0)
			PMD_DRV_LOG(ERR, "Failed to stop device on port %u",
				    vf_dev->data->port_id);
		if (ret == 0)
			ret = err;
```

This preserves the first error while still attempting to stop the VF.

Everything else looks correct. The lock upgrades from read to write are appropriate, the conditional logic in `hn_vf_add_unlocked()` correctly defers the datapath switch when the device isn't started, and `hn_vf_start()` properly rolls back the VF start on datapath switch failure.