Drivers: hv: vmbus: Replace the per-CPU channel lists with a global array of channels

When Hyper-V sends an interrupt to the guest, the guest has to figure
out which channel the interrupt is associated with.  Hyper-V sets a bit
in a memory page that is shared with the guest, indicating a particular
"relid" that the interrupt is associated with.  The current Linux code
then uses a set of per-CPU linked lists to map a given "relid" to a
pointer to a channel structure.

This design introduces a synchronization problem if the CPU that Hyper-V
will interrupt for a certain channel is changed.  If the interrupt comes
on the "old CPU" and the channel was already moved to the per-CPU list
of the "new CPU", then the relid -> channel mapping will fail and the
interrupt is dropped.  Similarly, if the interrupt comes on the new CPU
but the channel was not moved to the per-CPU list of the new CPU, then
the mapping will fail and the interrupt is dropped.

Relids are integers ranging from 0 to 2047.  The mapping from relids to
channel structures can be done by setting up an array with 2048 entries,
each entry being a pointer to a channel structure (hence total size ~16K
bytes, which is not a problem).  The array is global, so there are no
per-CPU linked lists to update.  The array can be searched and updated
by loading from/storing to the array at the specified index.  With no
per-CPU data structures, the above mentioned synchronization problem is
avoided and the relid2channel() function gets simpler.

Suggested-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20200406001514.19876-4-parri.andrea@gmail.com
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>

1 files changed, 8 insertions, 6 deletions

diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index 000740d053a2..3edf993b0fd9 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -132,12 +132,6 @@ struct hv_per_cpu_context {
 	 * basis.
 	 */
 	struct tasklet_struct msg_dpc;
-
-	/*
-	 * To optimize the mapping of relid to channel, maintain
-	 * per-cpu list of the channels based on their CPU affinity.
-	 */
-	struct list_head chan_list;
 };
 
 struct hv_context {
@@ -202,6 +196,8 @@ int hv_ringbuffer_read(struct vmbus_channel *channel,
 /* TODO: Need to make this configurable */
 #define MAX_NUM_CHANNELS_SUPPORTED	256
 
+#define MAX_CHANNEL_RELIDS					\
+	max(MAX_NUM_CHANNELS_SUPPORTED, HV_EVENT_FLAGS_COUNT)
 
 enum vmbus_connect_state {
 	DISCONNECTED,
@@ -251,6 +247,9 @@ struct vmbus_connection {
 	struct list_head chn_list;
 	struct mutex channel_mutex;
 
+	/* Array of channels */
+	struct vmbus_channel **channels;
+
 	/*
 	 * An offer message is handled first on the work_queue, and then
 	 * is further handled on handle_primary_chan_wq or
@@ -338,6 +337,9 @@ int vmbus_add_channel_kobj(struct hv_device *device_obj,
 
 void vmbus_remove_channel_attr_group(struct vmbus_channel *channel);
 
+void vmbus_channel_map_relid(struct vmbus_channel *channel);
+void vmbus_channel_unmap_relid(struct vmbus_channel *channel);
+
 struct vmbus_channel *relid2channel(u32 relid);
 
 void vmbus_free_channels(void);