scsi: storvsc: Fix calculation of sub-channel count

BugLink: https://bugs.launchpad.net/bugs/1830176

[ Upstream commit 382e06d1 ]

When the number of sub-channels offered by Hyper-V is >= the number of CPUs
in the VM, calculate the correct number of sub-channels.  The current code
produces one too many.

This scenario arises only when the number of CPUs is artificially
restricted (for example, with maxcpus=<n> on the kernel boot line), because
Hyper-V normally offers a sub-channel count < number of CPUs.  While the
current code doesn't break, the extra sub-channel is unbalanced across the
CPUs (for example, a total of 5 channels on a VM with 4 CPUs).
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/scsi/storvsc_drv.c

@@ -655,13 +655,22 @@ static void handle_sc_creation(struct vmbus_channel *new_sc)
 static void  handle_multichannel_storage(struct hv_device *device, int max_chns)
 {
 	struct storvsc_device *stor_device;
-	int num_cpus = num_online_cpus();
 	int num_sc;
 	struct storvsc_cmd_request *request;
 	struct vstor_packet *vstor_packet;
 	int ret, t;
 
-	num_sc = ((max_chns > num_cpus) ? num_cpus : max_chns);
+	/*
+	 * If the number of CPUs is artificially restricted, such as
+	 * with maxcpus=1 on the kernel boot line, Hyper-V could offer
+	 * sub-channels >= the number of CPUs. These sub-channels
+	 * should not be created. The primary channel is already created
+	 * and assigned to one CPU, so check against # CPUs - 1.
+	 */
+	num_sc = min((int)(num_online_cpus() - 1), max_chns);
+	if (!num_sc)
+		return;
+
 	stor_device = get_out_stor_device(device);
 	if (!stor_device)
 		return;