[PATCH v3 3/3] Drivers: hv: vmbus: Check for pending channel interrupts before taking a CPU offline

From: Andrea Parri (Microsoft)
Date: Fri Apr 16 2021 - 10:35:27 EST


Check that enough time has passed such that the modify channel message
has been processed before taking a CPU offline.

Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
---
drivers/hv/hv.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 53 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/hv.c b/drivers/hv/hv.c
index 3e6ff83adff42..e0c522d143a37 100644
--- a/drivers/hv/hv.c
+++ b/drivers/hv/hv.c
@@ -15,6 +15,7 @@
#include <linux/hyperv.h>
#include <linux/random.h>
#include <linux/clockchips.h>
+#include <linux/delay.h>
#include <linux/interrupt.h>
#include <clocksource/hyperv_timer.h>
#include <asm/mshyperv.h>
@@ -292,12 +293,50 @@ void hv_synic_disable_regs(unsigned int cpu)
disable_percpu_irq(vmbus_irq);
}

+#define HV_MAX_TRIES 3
+/*
+ * Scan the event flags page of 'this' CPU looking for any bit that is set. If we find one
+ * bit set, then wait for a few milliseconds. Repeat these steps for a maximum of 3 times.
+ * Return 'true', if there is still any set bit after this operation; 'false', otherwise.
+ *
+ * If a bit is set, that means there is a pending channel interrupt. The expectation is
+ * that the normal interrupt handling mechanism will find and process the channel interrupt
+ * "very soon", and in the process clear the bit.
+ */
+static bool hv_synic_event_pending(void)
+{
+ struct hv_per_cpu_context *hv_cpu = this_cpu_ptr(hv_context.cpu_context);
+ union hv_synic_event_flags *event =
+ (union hv_synic_event_flags *)hv_cpu->synic_event_page + VMBUS_MESSAGE_SINT;
+ unsigned long *recv_int_page = event->flags; /* assumes VMBus version >= VERSION_WIN8 */
+ bool pending;
+ u32 relid;
+ int tries = 0;
+
+retry:
+ pending = false;
+ for_each_set_bit(relid, recv_int_page, HV_EVENT_FLAGS_COUNT) {
+ /* Special case - VMBus channel protocol messages */
+ if (relid == 0)
+ continue;
+ pending = true;
+ break;
+ }
+ if (pending && tries++ < HV_MAX_TRIES) {
+ usleep_range(10000, 20000);
+ goto retry;
+ }
+ return pending;
+}

int hv_synic_cleanup(unsigned int cpu)
{
struct vmbus_channel *channel, *sc;
bool channel_found = false;

+ if (vmbus_connection.conn_state != CONNECTED)
+ goto always_cleanup;
+
/*
* Hyper-V does not provide a way to change the connect CPU once
* it is set; we must prevent the connect CPU from going offline
@@ -305,8 +344,7 @@ int hv_synic_cleanup(unsigned int cpu)
* path where the vmbus is already disconnected, the CPU must be
* allowed to shut down.
*/
- if (cpu == VMBUS_CONNECT_CPU &&
- vmbus_connection.conn_state == CONNECTED)
+ if (cpu == VMBUS_CONNECT_CPU)
return -EBUSY;

/*
@@ -333,9 +371,21 @@ int hv_synic_cleanup(unsigned int cpu)
}
mutex_unlock(&vmbus_connection.channel_mutex);

- if (channel_found && vmbus_connection.conn_state == CONNECTED)
+ 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
+ * vmbus_send_modifychannel(). Scan the event flags page looking for
+ * bits that are set and waiting with a timeout for vmbus_chan_sched()
+ * to process such bits. If bits are still set after this operation
+ * and VMBus is connected, fail the CPU offlining operation.
+ */
+ if (vmbus_proto_version >= VERSION_WIN10_V4_1 && hv_synic_event_pending())
return -EBUSY;

+always_cleanup:
hv_stimer_legacy_cleanup(cpu);

hv_synic_disable_regs(cpu);
--
2.25.1