[PATCH] Drivers: hv: vmbus: handle various crash scenarios

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Kdump keeps biting. Turns out CHANNELMSG_UNLOAD_RESPONSE is always
delivered to CPU0 regardless of what CPU we're sending CHANNELMSG_UNLOAD
from. vmbus_wait_for_unload() doesn't account for the fact that in case
we're crashing on some other CPU and CPU0 is still alive and operational
CHANNELMSG_UNLOAD_RESPONSE will be delivered there completing
vmbus_connection.unload_event, our wait on the current CPU will never
end.

Do the following:
1) Check for completion_done() in the loop. In case interrupt handler is
   still alive we'll get the confirmation we need.

2) Always read CPU0's message page as CHANNELMSG_UNLOAD_RESPONSE will be
   delivered there. We can race with still-alive interrupt handler doing
   the same but we don't care as we're checking completion_done() now.

3) Cleanup message pages on all CPUs. This is required (at least for the
   current CPU as we're clearing CPU0 messages now but we may want to bring
   up additional CPUs on crash) as new messages won't be delivered till we
   consume what's pending. On boot we'll place message pages somewhere else
   and we won't be able to read stale messages.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 drivers/hv/channel_mgmt.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index b10e8f74..5f37057 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -512,14 +512,26 @@ static void init_vp_index(struct vmbus_channel *channel, const uuid_le *type_gui
 
 static void vmbus_wait_for_unload(void)
 {
-	int cpu = smp_processor_id();
-	void *page_addr = hv_context.synic_message_page[cpu];
+	int cpu;
+	void *page_addr = hv_context.synic_message_page[0];
 	struct hv_message *msg = (struct hv_message *)page_addr +
 				  VMBUS_MESSAGE_SINT;
 	struct vmbus_channel_message_header *hdr;
 	bool unloaded = false;
 
-	while (1) {
+	/*
+	 * CHANNELMSG_UNLOAD_RESPONSE is always delivered to CPU0. When we're
+	 * crashing on a different CPU let's hope that IRQ handler on CPU0 is
+	 * still functional and vmbus_unload_response() will complete
+	 * vmbus_connection.unload_event. If not, the last thing we can do is
+	 * read message page for CPU0 regardless of what CPU we're on.
+	 */
+	while (!unloaded) {
+		if (completion_done(&vmbus_connection.unload_event)) {
+			unloaded = true;
+			break;
+		}
+
 		if (READ_ONCE(msg->header.message_type) == HVMSG_NONE) {
 			mdelay(10);
 			continue;
@@ -530,9 +542,17 @@ static void vmbus_wait_for_unload(void)
 			unloaded = true;
 
 		vmbus_signal_eom(msg);
+	}
 
-		if (unloaded)
-			break;
+	/*
+	 * We're crashing and already got the UNLOAD_RESPONSE, cleanup all
+	 * maybe-pending messages on all CPUs to be able to receive new
+	 * messages after we reconnect.
+	 */
+	for_each_online_cpu(cpu) {
+		page_addr = hv_context.synic_message_page[cpu];
+		msg = (struct hv_message *)page_addr + VMBUS_MESSAGE_SINT;
+		msg->header.message_type = HVMSG_NONE;
 	}
 }
 
-- 
2.5.0