[PATCH 09/10] xen/blkback: get hardware queue number from blkfront

[Bob Liu <bob.liu@oracle.com>]

Backend advertise the max number it supported to "multi-queue-max-queues", and
read the negotiate value from "multi-queue-num-queues".

Signed-off-by: Bob Liu <bob.liu@oracle.com>
---
 drivers/block/xen-blkback/blkback.c |  8 ++++++++
 drivers/block/xen-blkback/xenbus.c  | 36 ++++++++++++++++++++++++++++++------
 2 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 0969e7e..34d72b0 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -80,6 +80,11 @@ module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
 MODULE_PARM_DESC(max_persistent_grants,
                  "Maximum number of grants to map persistently");
 
+unsigned int xenblk_max_queues;
+module_param_named(max_queues, xenblk_max_queues, uint, 0644);
+MODULE_PARM_DESC(max_queues,
+		 "Maximum number of hardware queues per virtual disk");
+
 /*
  * The LRU mechanism to clean the lists of persistent grants needs to
  * be executed periodically. The time interval between consecutive executions
@@ -1390,6 +1395,9 @@ static int __init xen_blkif_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+	/* Allow as many queues as there are CPUs, by default */
+	xenblk_max_queues = num_online_cpus();
+
 	rc = xen_blkif_interface_init();
 	if (rc)
 		goto failed_init;
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 93e5f38..c33d8c9 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -21,6 +21,8 @@
 #include <xen/grant_table.h>
 #include "common.h"
 
+extern unsigned int xenblk_max_queues;
+
 struct backend_info {
 	struct xenbus_device	*dev;
 	struct xen_blkif	*blkif;
@@ -225,12 +227,6 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 	atomic_set(&blkif->drain, 0);
 	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
 
-	blkif->nr_rings = 1;
-	if (xen_blkif_alloc_rings(blkif)) {
-		kmem_cache_free(xen_blkif_cachep, blkif);
-		return ERR_PTR(-ENOMEM);
-	}
-
 	return blkif;
 }
 
@@ -647,6 +643,14 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
 		goto fail;
 	}
 
+	/* Multi-queue support: This is an optional feature. */
+	err = xenbus_printf(XBT_NIL, dev->nodename,
+			    "multi-queue-max-queues", "%u", xenblk_max_queues);
+	if (err) {
+		pr_debug("Error writing multi-queue-num-queues\n");
+		goto fail;
+	}
+
 	/* setup back pointer */
 	be->blkif->be = be;
 
@@ -927,6 +931,7 @@ static int connect_ring(struct backend_info *be)
 	char *xspath;
 	size_t xspathsize;
 	const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
+	unsigned int requested_num_queues = 0;
 
 	DPRINTK("%s", dev->otherend);
 
@@ -954,6 +959,25 @@ static int connect_ring(struct backend_info *be)
 	be->blkif->vbd.feature_gnt_persistent = pers_grants;
 	be->blkif->vbd.overflow_max_grants = 0;
 
+	/*
+	 * Read the number of hardware queus from frontend.
+	 */
+	err = xenbus_scanf(XBT_NIL, dev->otherend, "multi-queue-num-queues", "%u", &requested_num_queues);
+	if (err < 0) {
+		requested_num_queues = 1;
+	} else {
+		if (requested_num_queues > xenblk_max_queues) {
+			/* buggy or malicious guest */
+			xenbus_dev_fatal(dev, err,
+					"guest requested %u queues, exceeding the maximum of %u.",
+					requested_num_queues, xenblk_max_queues);
+			return -1;
+		}
+	}
+	be->blkif->nr_rings = requested_num_queues;
+	if (xen_blkif_alloc_rings(be->blkif))
+		return -ENOMEM;
+
 	if (be->blkif->nr_rings == 1) {
 		err = xenbus_gather(XBT_NIL, dev->otherend, "ring-ref", "%lu",
 				&ring_ref, "event-channel", "%u", &evtchn, NULL);
-- 
1.8.3.1