block backends: simplify protocol negotiation and initialization

In the negotiation code we can use the simpler (and type safe)
xenbus_read() instead of xenbus_gather() for reading the "protocol"
node.

The initialization code can be considerably shrunk by using a macro to
abstract out the code common for all variants.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/drivers/xen/blkback/interface.c
+++ b/drivers/xen/blkback/interface.c
@@ -70,29 +70,22 @@ int blkif_map(blkif_t *blkif, grant_ref_
 	blkif->blk_ring_area = area;
 
 	switch (blkif->blk_protocol) {
+#define BLKBK_RING_INIT(p) ({ \
+		struct blkif_##p##_sring *sring = area->addr; \
+		BACK_RING_INIT(&blkif->blk_rings.p, sring, PAGE_SIZE); \
+	})
 	case BLKIF_PROTOCOL_NATIVE:
-	{
-		blkif_sring_t *sring;
-		sring = (blkif_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE);
+		BLKBK_RING_INIT(native);
 		break;
-	}
 	case BLKIF_PROTOCOL_X86_32:
-	{
-		blkif_x86_32_sring_t *sring_x86_32;
-		sring_x86_32 = (blkif_x86_32_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE);
+		BLKBK_RING_INIT(x86_32);
 		break;
-	}
 	case BLKIF_PROTOCOL_X86_64:
-	{
-		blkif_x86_64_sring_t *sring_x86_64;
-		sring_x86_64 = (blkif_x86_64_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE);
+		BLKBK_RING_INIT(x86_64);
 		break;
-	}
 	default:
 		BUG();
+#undef BLKBK_RING_INIT
 	}
 
 	err = bind_interdomain_evtchn_to_irqhandler(
--- a/drivers/xen/blkback/xenbus.c
+++ b/drivers/xen/blkback/xenbus.c
@@ -494,17 +494,14 @@ static int connect_ring(struct backend_i
 	}
 
 	be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
-	err = xenbus_gather(XBT_NIL, dev->otherend, "protocol",
-			    NULL, &protocol, NULL);
-	if (err)
+	protocol = xenbus_read(XBT_NIL, dev->otherend, "protocol", NULL);
+	if (IS_ERR(protocol))
 		protocol = NULL;
-	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE))
-		be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_32))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_64))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
-	else {
+	else if (0 != strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE)) {
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		kfree(protocol);
 		return -1;
--- a/drivers/xen/blktap/interface.c
+++ b/drivers/xen/blktap/interface.c
@@ -71,29 +71,22 @@ int tap_blkif_map(blkif_t *blkif, struct
 	blkif->blk_ring_area = area;
 
 	switch (blkif->blk_protocol) {
+#define BLKTAP_RING_INIT(p) ({ \
+		struct blkif_##p##_sring *sring = area->addr; \
+		BACK_RING_INIT(&blkif->blk_rings.p, sring, PAGE_SIZE); \
+	})
 	case BLKIF_PROTOCOL_NATIVE:
-	{
-		blkif_sring_t *sring;
-		sring = (blkif_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.native, sring, PAGE_SIZE);
+		BLKTAP_RING_INIT(native);
 		break;
-	}
 	case BLKIF_PROTOCOL_X86_32:
-	{
-		blkif_x86_32_sring_t *sring_x86_32;
-		sring_x86_32 = (blkif_x86_32_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.x86_32, sring_x86_32, PAGE_SIZE);
+		BLKTAP_RING_INIT(x86_32);
 		break;
-	}
 	case BLKIF_PROTOCOL_X86_64:
-	{
-		blkif_x86_64_sring_t *sring_x86_64;
-		sring_x86_64 = (blkif_x86_64_sring_t *)area->addr;
-		BACK_RING_INIT(&blkif->blk_rings.x86_64, sring_x86_64, PAGE_SIZE);
+		BLKTAP_RING_INIT(x86_64);
 		break;
-	}
 	default:
 		BUG();
+#undef BLKTAP_RING_INIT
 	}
 
 	err = bind_interdomain_evtchn_to_irqhandler(
--- a/drivers/xen/blktap/xenbus.c
+++ b/drivers/xen/blktap/xenbus.c
@@ -447,17 +447,14 @@ static int connect_ring(struct backend_i
 	}
 
 	be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
-	err = xenbus_gather(XBT_NIL, dev->otherend, "protocol",
-			    NULL, &protocol, NULL);
-	if (err)
+	protocol = xenbus_read(XBT_NIL, dev->otherend, "protocol", NULL);
+	if (IS_ERR(protocol))
 		protocol = NULL;
-	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE))
-		be->blkif->blk_protocol = BLKIF_PROTOCOL_NATIVE;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_32))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_32;
 	else if (0 == strcmp(protocol, XEN_IO_PROTO_ABI_X86_64))
 		be->blkif->blk_protocol = BLKIF_PROTOCOL_X86_64;
-	else {
+	else if (0 != strcmp(protocol, XEN_IO_PROTO_ABI_NATIVE)) {
 		xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol);
 		kfree(protocol);
 		return -1;
--- a/include/xen/blkif.h
+++ b/include/xen/blkif.h
@@ -72,6 +72,7 @@ typedef struct blkif_x86_64_request blki
 typedef struct blkif_x86_64_request blkif_x86_64_request_t;
 typedef struct blkif_x86_64_response blkif_x86_64_response_t;
 
+#define blkif_native_sring blkif_sring
 DEFINE_RING_TYPES(blkif_common, struct blkif_common_request, struct blkif_common_response);
 DEFINE_RING_TYPES(blkif_x86_32, struct blkif_x86_32_request, struct blkif_x86_32_response);
 DEFINE_RING_TYPES(blkif_x86_64, struct blkif_x86_64_request, struct blkif_x86_64_response);