[PATCH v2 2/2] drivers/hv: add CPU offlining support

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

Currently, it is effectively impossible to offline CPUs. Since, most
CPUs will have vmbus channels attached to them. So, as made mention of
in commit d570aec0f2154 ("Drivers: hv: vmbus: Synchronize
init_vp_index() vs. CPU hotplug"), rebind channels associated with CPUs
that a user is trying to offline to a new "randomly" selected CPU.

Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
v2: remove cpus_read_{un,}lock() from hv_pick_new_cpu() and add
    lockdep_assert_cpus_held().
---
 drivers/hv/hv.c | 56 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 36d9ba097ff5..9fef71403c86 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -433,13 +433,39 @@ static bool hv_synic_event_pending(void)
 	return pending;
 }
 
+static int hv_pick_new_cpu(struct vmbus_channel *channel,
+			   unsigned int current_cpu)
+{
+	int ret = 0;
+	int cpu;
+
+	lockdep_assert_cpus_held();
+	lockdep_assert_held(&vmbus_connection.channel_mutex);
+
+	/*
+	 * We can't assume that the relevant interrupts will be sent before
+	 * the cpu is offlined on older versions of hyperv.
+	 */
+	if (vmbus_proto_version < VERSION_WIN10_V5_3)
+		return -EBUSY;
+
+	cpu = cpumask_next(get_random_u32_below(nr_cpu_ids), cpu_online_mask);
+
+	if (cpu >= nr_cpu_ids || cpu == current_cpu)
+		cpu = VMBUS_CONNECT_CPU;
+
+	ret = vmbus_channel_set_cpu(channel, cpu);
+
+	return ret;
+}
+
 /*
  * hv_synic_cleanup - Cleanup routine for hv_synic_init().
  */
 int hv_synic_cleanup(unsigned int cpu)
 {
 	struct vmbus_channel *channel, *sc;
-	bool channel_found = false;
+	int ret = 0;
 
 	if (vmbus_connection.conn_state != CONNECTED)
 		goto always_cleanup;
@@ -456,31 +482,31 @@ int hv_synic_cleanup(unsigned int cpu)
 
 	/*
 	 * Search for channels which are bound to the CPU we're about to
-	 * cleanup.  In case we find one and vmbus is still connected, we
-	 * fail; this will effectively prevent CPU offlining.
-	 *
-	 * TODO: Re-bind the channels to different CPUs.
+	 * cleanup.
 	 */
 	mutex_lock(&vmbus_connection.channel_mutex);
 	list_for_each_entry(channel, &vmbus_connection.chn_list, listentry) {
 		if (channel->target_cpu == cpu) {
-			channel_found = true;
-			break;
+			ret = hv_pick_new_cpu(channel, cpu);
+
+			if (ret) {
+				mutex_unlock(&vmbus_connection.channel_mutex);
+				return ret;
+			}
 		}
 		list_for_each_entry(sc, &channel->sc_list, sc_list) {
 			if (sc->target_cpu == cpu) {
-				channel_found = true;
-				break;
+				ret = hv_pick_new_cpu(channel, cpu);
+
+				if (ret) {
+					mutex_unlock(&vmbus_connection.channel_mutex);
+					return ret;
+				}
 			}
 		}
-		if (channel_found)
-			break;
 	}
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
-	if (channel_found)
-		return -EBUSY;
-
 	/*
 	 * channel_found == false means that any channels that were previously
 	 * assigned to the CPU have been reassigned elsewhere with a call of
@@ -497,5 +523,5 @@ int hv_synic_cleanup(unsigned int cpu)
 
 	hv_synic_disable_regs(cpu);
 
-	return 0;
+	return ret;
 }
-- 
2.47.1