[Xen-devel] [PATCH RFC v2 5/5] xen, blkback: negotiate of the number of block rings with the frontend

[Arianna Avanzini <avanzini.arianna@xxxxxxxxx>]

This commit lets the backend driver advertise the number of available
hardware queues; it also implements gathering from the frontend driver
the number of rings actually available for mapping.

Signed-off-by: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
---
 drivers/block/xen-blkback/xenbus.c | 44 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkback/xenbus.c 
b/drivers/block/xen-blkback/xenbus.c
index a4f13cc..9ff6ced 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -477,6 +477,34 @@ static void xen_vbd_free(struct xen_vbd *vbd)
        vbd->bdev = NULL;
 }
 
+static int xen_advertise_hw_queues(struct xen_blkif *blkif,
+                                  struct request_queue *q)
+{
+       struct xen_vbd *vbd = &blkif->vbd;
+       struct xenbus_transaction xbt;
+       int err;
+
+       if (q && q->mq_ops)
+               vbd->nr_supported_hw_queues = q->nr_hw_queues;
+
+       err = xenbus_transaction_start(&xbt);
+       if (err) {
+               BUG_ON(!blkif->be);
+               xenbus_dev_fatal(blkif->be->dev, err, "starting transaction (hw 
queues)");
+               return err;
+       }
+
+       err = xenbus_printf(xbt, blkif->be->dev->nodename, 
"nr_supported_hw_queues", "%u",
+                           blkif->vbd.nr_supported_hw_queues);
+       if (err)
+               xenbus_dev_error(blkif->be->dev, err, "writing 
%s/nr_supported_hw_queues",
+                                blkif->be->dev->nodename);
+
+       xenbus_transaction_end(xbt, 0);
+
+       return err;
+}
+
 static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
                          unsigned major, unsigned minor, int readonly,
                          int cdrom)
@@ -484,6 +512,7 @@ static int xen_vbd_create(struct xen_blkif *blkif, 
blkif_vdev_t handle,
        struct xen_vbd *vbd;
        struct block_device *bdev;
        struct request_queue *q;
+       int err;
 
        vbd = &blkif->vbd;
        vbd->handle   = handle;
@@ -522,6 +551,10 @@ static int xen_vbd_create(struct xen_blkif *blkif, 
blkif_vdev_t handle,
        if (q && blk_queue_secdiscard(q))
                vbd->discard_secure = true;
 
+       err = xen_advertise_hw_queues(blkif, q);
+       if (err)
+               return -ENOENT;
+
        DPRINTK("Successful creation of handle=%04x (dom=%u)\n",
                handle, blkif->domid);
        return 0;
@@ -935,7 +968,16 @@ static int connect_ring(struct backend_info *be)
 
        DPRINTK("%s", dev->otherend);
 
-       blkif->nr_rings = 1;
+       err = xenbus_gather(XBT_NIL, dev->otherend, "nr_blk_rings",
+                           "%u", &blkif->nr_rings, NULL);
+       if (err) {
+               /*
+                * Frontend does not support multiqueue; force compatibility
+                * mode of the driver.
+                */
+               blkif->vbd.nr_supported_hw_queues = 0;
+               blkif->nr_rings = 1;
+       }
 
        ring_ref = kzalloc(sizeof(unsigned long) * blkif->nr_rings, GFP_KERNEL);
        if (!ring_ref)
-- 
2.1.0


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