Subject: Re: [Xen-devel] xen-netback: make feature-rx-notify mandatory -- Breaks stubdoms

From: David Vrabel <[email-masked]>
Date: Wed, 17 Dec 2014 14:00:41 +0000

On 10/12/14 14:12, David Vrabel wrote:
> On 10/12/14 13:42, John wrote:
>> David,
>>
>> This patch you put into 3.18.0 appears to break the latest version of
>> stubdomains. I found this out today when I tried to update a machine to
>> 3.18.0 and all of the domUs crashed on start with the dmesg output like
>> this:
> 
> Cc'ing the lists and relevant netback maintainers.
> 
> I guess the stubdoms are using minios's netfront?  This is something I
> forgot about when deciding if it was ok to make this feature mandatory.
> 
> The patch cannot be reverted as it's a prerequisite for a critical
> (security) bug fix.  I am also unconvinced that the no-feature-rx-notify
> support worked correctly anyway.
> 
> This can be resolved by:
> 
> - Fixing minios's netfront to support feature-rx-notify. This should be
> easy but wouldn't help existing Xen deployments.
> 
> Or:
> 
> - Reimplement feature-rx-notify support.  I think the easiest way is to
> queue packets on the guest Rx internal queue with a short expiry time.

This patch works for me.  I tested it with a hacked Linux frontend that
disabled feature-rx-notify, but not with a stubdom.

Can you give it a try, please?

David

8<--------------------------------------------------------------
xen-netback: support frontends without feature-rx-notify again

Commit bc96f648df1bbc2729abbb84513cf4f64273a1f1 (xen-netback: make
feature-rx-notify mandatory) incorrectly assumed that there were no
frontends in use that did not support this feature.  But the frontend
driver in MiniOS does not and since this is used by (qemu) stubdoms,
these stopped working.

Netback sort of works as-is in this mode except:

- If there are no Rx requests and the internal Rx queue fills, only the
  drain timeout will wake the thread.  The default drain timeout of 10 s
  would give unacceptable pauses.

- If an Rx stall was detected and the internal Rx queue is drained, then
  the Rx thread would never wake.

Handle these two cases (when feature-rx-notify is disabled) by:

- Reducing the drain timeout to 30 ms.

- Disabling Rx stall detection.

Signed-off-by: David Vrabel <[email-masked]>
---
 drivers/net/xen-netback/common.h    |    4 +++-
 drivers/net/xen-netback/interface.c |    4 +++-
 drivers/net/xen-netback/netback.c   |   27 ++++++++++++++-------------
 drivers/net/xen-netback/xenbus.c    |   12 +++++++++---
 4 files changed, 29 insertions(+), 18 deletions(-)

diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
index 083ecc9..5f1fda4 100644
--- a/drivers/net/xen-netback/common.h
+++ b/drivers/net/xen-netback/common.h
@@ -230,6 +230,8 @@ struct xenvif {
 	 */
 	bool disabled;
 	unsigned long status;
+	unsigned long drain_timeout;
+	unsigned long stall_timeout;
 
 	/* Queues */
 	struct xenvif_queue *queues;
@@ -328,7 +330,7 @@ irqreturn_t xenvif_interrupt(int irq, void *dev_id);
 extern bool separate_tx_rx_irq;
 
 extern unsigned int rx_drain_timeout_msecs;
-extern unsigned int rx_drain_timeout_jiffies;
+extern unsigned int rx_stall_timeout_msecs;
 extern unsigned int xenvif_max_queues;
 
 #ifdef CONFIG_DEBUG_FS
diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index a6a32d3..9259a73 100644
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -166,7 +166,7 @@ static int xenvif_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		goto drop;
 
 	cb = XENVIF_RX_CB(skb);
-	cb->expires = jiffies + rx_drain_timeout_jiffies;
+	cb->expires = jiffies + vif->drain_timeout;
 
 	xenvif_rx_queue_tail(queue, skb);
 	xenvif_kick_thread(queue);
@@ -414,6 +414,8 @@ struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
 	vif->ip_csum = 1;
 	vif->dev = dev;
 	vif->disabled = false;
+	vif->drain_timeout = msecs_to_jiffies(rx_drain_timeout_msecs);
+	vif->stall_timeout = msecs_to_jiffies(rx_stall_timeout_msecs);
 
 	/* Start out with no queues. */
 	vif->queues = NULL;
diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 4a509f7..b0292e4 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -60,14 +60,12 @@ module_param(separate_tx_rx_irq, bool, 0644);
  */
 unsigned int rx_drain_timeout_msecs = 10000;
 module_param(rx_drain_timeout_msecs, uint, 0444);
-unsigned int rx_drain_timeout_jiffies;
 
 /* The length of time before the frontend is considered unresponsive
  * because it isn't providing Rx slots.
  */
-static unsigned int rx_stall_timeout_msecs = 60000;
+unsigned int rx_stall_timeout_msecs = 60000;
 module_param(rx_stall_timeout_msecs, uint, 0444);
-static unsigned int rx_stall_timeout_jiffies;
 
 unsigned int xenvif_max_queues;
 module_param_named(max_queues, xenvif_max_queues, uint, 0644);
@@ -2020,7 +2018,7 @@ static bool xenvif_rx_queue_stalled(struct xenvif_queue *queue)
 	return !queue->stalled
 		&& prod - cons < XEN_NETBK_RX_SLOTS_MAX
 		&& time_after(jiffies,
-			      queue->last_rx_time + rx_stall_timeout_jiffies);
+			      queue->last_rx_time + queue->vif->stall_timeout);
 }
 
 static bool xenvif_rx_queue_ready(struct xenvif_queue *queue)
@@ -2038,8 +2036,9 @@ static bool xenvif_have_rx_work(struct xenvif_queue *queue)
 {
 	return (!skb_queue_empty(&queue->rx_queue)
 		&& xenvif_rx_ring_slots_available(queue, XEN_NETBK_RX_SLOTS_MAX))
-		|| xenvif_rx_queue_stalled(queue)
-		|| xenvif_rx_queue_ready(queue)
+		|| (queue->vif->stall_timeout &&
+		    (xenvif_rx_queue_stalled(queue)
+		     || xenvif_rx_queue_ready(queue)))
 		|| kthread_should_stop()
 		|| queue->vif->disabled;
 }
@@ -2092,6 +2091,9 @@ int xenvif_kthread_guest_rx(void *data)
 	struct xenvif_queue *queue = data;
 	struct xenvif *vif = queue->vif;
 
+	if (!vif->stall_timeout)
+		xenvif_queue_carrier_on(queue);
+
 	for (;;) {
 		xenvif_wait_for_rx_work(queue);
 
@@ -2118,10 +2120,12 @@ int xenvif_kthread_guest_rx(void *data)
 		 * while it's probably not responsive, drop the
 		 * carrier so packets are dropped earlier.
 		 */
-		if (xenvif_rx_queue_stalled(queue))
-			xenvif_queue_carrier_off(queue);
-		else if (xenvif_rx_queue_ready(queue))
-			xenvif_queue_carrier_on(queue);
+		if (queue->vif->stall_timeout) {
+			if (xenvif_rx_queue_stalled(queue))
+				xenvif_queue_carrier_off(queue);
+			else if (xenvif_rx_queue_ready(queue))
+				xenvif_queue_carrier_on(queue);
+		}
 
 		/* Queued packets may have foreign pages from other
 		 * domains.  These cannot be queued indefinitely as
@@ -2192,9 +2196,6 @@ static int __init netback_init(void)
 	if (rc)
 		goto failed_init;
 
-	rx_drain_timeout_jiffies = msecs_to_jiffies(rx_drain_timeout_msecs);
-	rx_stall_timeout_jiffies = msecs_to_jiffies(rx_stall_timeout_msecs);
-
 #ifdef CONFIG_DEBUG_FS
 	xen_netback_dbg_root = debugfs_create_dir("xen-netback", NULL);
 	if (IS_ERR_OR_NULL(xen_netback_dbg_root))
diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index d44cd19..efbaf2a 100644
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -887,9 +887,15 @@ static int read_xenbus_vif_flags(struct backend_info *be)
 		return -EOPNOTSUPP;
 
 	if (xenbus_scanf(XBT_NIL, dev->otherend,
-			 "feature-rx-notify", "%d", &val) < 0 || val == 0) {
-		xenbus_dev_fatal(dev, -EINVAL, "feature-rx-notify is mandatory");
-		return -EINVAL;
+			 "feature-rx-notify", "%d", &val) < 0)
+		val = 0;
+	if (!val) {
+		/* - Reduce drain timeout to poll more frequently for
+		 *   Rx requests.
+		 * - Disable Rx stall detection.
+		 */
+		be->vif->drain_timeout = msecs_to_jiffies(30);
+		be->vif->stall_timeout = 0;
 	}
 
 	if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-sg",
-- 
1.7.10.4