xen/blkback: get the number of hardware queues/rings from blkfront

Backend advertises "multi-queue-max-queues" to front, also get the negotiated
number from "multi-queue-num-queues" written by blkfront.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

3 files changed, 42 insertions, 6 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 4fd8640d146c..18b27770d80b 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -84,6 +84,16 @@ MODULE_PARM_DESC(max_persistent_grants,
                  "Maximum number of grants to map persistently");
 
 /*
+ * Maximum number of rings/queues blkback supports, allow as many queues as there
+ * are CPUs if user has not specified a value.
+ */
+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." \
+                 "By default it is the number of online CPUs.");
+
+/*
  * Maximum order of pages to be used for the shared ring between front and
  * backend, 4KB page granularity is used.
  */
@@ -1483,6 +1493,9 @@ static int __init xen_blkif_init(void)
                 xen_blkif_max_ring_order = XENBUS_MAX_RING_GRANT_ORDER;
         }
 
+        if (xenblk_max_queues == 0)
+                xenblk_max_queues = num_online_cpus();
+
         rc = xen_blkif_interface_init();
         if (rc)
                 goto failed_init;
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 310eff3cf43f..847444dc1df4 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -46,6 +46,7 @@
 #include <xen/interface/io/protocols.h>
 
 extern unsigned int xen_blkif_max_ring_order;
+extern unsigned int xenblk_max_queues;
 /*
  * This is the maximum number of segments that would be allowed in indirect
  * requests. This value will also be passed to the frontend.
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index f5bfedd0e948..0d6bb9383a68 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -181,12 +181,6 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
         blkif->st_print = jiffies;
         INIT_WORK(&blkif->persistent_purge_work, xen_blkbk_unmap_purged_grants);
 
-        blkif->nr_rings = 1;
-        if (xen_blkif_alloc_rings(blkif)) {
-                kmem_cache_free(xen_blkif_cachep, blkif);
-                return ERR_PTR(-ENOMEM);
-        }
-
         return blkif;
 }
 
@@ -595,6 +589,12 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
                 goto fail;
         }
 
+        /* Multi-queue: advertise how many queues are supported by us.*/
+        err = xenbus_printf(XBT_NIL, dev->nodename,
+                            "multi-queue-max-queues", "%u", xenblk_max_queues);
+        if (err)
+                pr_warn("Error writing multi-queue-max-queues\n");
+
         /* setup back pointer */
         be->blkif->be = be;
 
@@ -980,6 +980,7 @@ 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;
 
         pr_debug("%s %s\n", __func__, dev->otherend);
 
@@ -1007,6 +1008,27 @@ 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 queues 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
+                    || requested_num_queues == 0) {
+                        /* 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 -ENOSYS;
+                }
+        }
+        be->blkif->nr_rings = requested_num_queues;
+        if (xen_blkif_alloc_rings(be->blkif))
+                return -ENOMEM;
+
         pr_info("%s: using %d queues, protocol %d (%s) %s\n", dev->nodename,
                  be->blkif->nr_rings, be->blkif->blk_protocol, protocol,
                  pers_grants ? "persistent grants" : "");