[PATCH RFC 38/39] xen-blkback: xen_shim_domain() support

[Joao Martins]

With xen_shim_domain() enabled, the struct pages used for grant mappings
are only valid in the interval between GNTTABOP_map_grant_ref and
GNTTABOP_unmap_grant_ref.

Ensure that we do not cache the struct page outside that duration.

Also, update the struct page for the segment after
GNTTABOP_map_grant_ref, for the subsequent tracking or when doing IO.

In addition, we disable persistent grants for this configuration, since
the map/unmap overhead is fairly small (in the fast path, page lookup,
get_page() and put_page().) This reduces the memory usage in
blkback/blkfront.

Co-developed-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 drivers/block/xen-blkback/blkback.c | 19 ++++++++++++++++---
 drivers/block/xen-blkback/xenbus.c  |  5 +++--
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index d51d88be88e1..20c15e6787b1 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -167,6 +167,15 @@ static inline void put_free_pages(struct xen_blkif_ring *ring, struct page **pag
 	unsigned long flags;
 	int i;
 
+	/*
+	 * We don't own the struct page after unmap has been called.
+	 * Reallocation is cheap anyway for the shim domain case, so free it.
+	 */
+	if (xen_shim_domain()) {
+		gnttab_free_pages(num, page);
+		return;
+	}
+
 	spin_lock_irqsave(&ring->free_pages_lock, flags);
 	for (i = 0; i < num; i++)
 		list_add(&page[i]->lru, &ring->free_pages);
@@ -825,7 +834,7 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 	 */
 again:
 	for (i = map_until; i < num; i++) {
-		uint32_t flags;
+		uint32_t flags = 0;
 
 		if (use_persistent_gnts) {
 			persistent_gnt = get_persistent_gnt(
@@ -846,7 +855,8 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 			addr = vaddr(pages[i]->page);
 			pages_to_gnt[segs_to_map] = pages[i]->page;
 			pages[i]->persistent_gnt = NULL;
-			flags = GNTMAP_host_map;
+			if (!xen_shim_domain())
+				flags = GNTMAP_host_map;
 			if (!use_persistent_gnts && ro)
 				flags |= GNTMAP_readonly;
 			gnttab_set_map_op(&map[segs_to_map++], addr,
@@ -880,6 +890,8 @@ static int xen_blkbk_map(struct xen_blkif_ring *ring,
 				goto next;
 			}
 			pages[seg_idx]->handle = map[new_map_idx].handle;
+			if (xen_shim_domain())
+				pages[seg_idx]->page = pages_to_gnt[new_map_idx];
 		} else {
 			continue;
 		}
@@ -1478,7 +1490,7 @@ static int __init xen_blkif_init(void)
 {
 	int rc = 0;
 
-	if (!xen_domain())
+	if (!xen_domain() && !xen_shim_domain_get())
 		return -ENODEV;
 
 	if (xen_blkif_max_ring_order > XENBUS_MAX_RING_GRANT_ORDER) {
@@ -1508,6 +1520,7 @@ static void __exit xen_blkif_exit(void)
 {
 	xen_blkif_interface_exit();
 	xen_blkif_xenbus_exit();
+	xen_shim_domain_put();
 }
 
 module_exit(xen_blkif_exit);
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 424e2efebe85..11be0c052b77 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -871,7 +871,8 @@ static void connect(struct backend_info *be)
 
 	xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
 
-	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1);
+	err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u",
+			    !xen_shim_domain());
 	if (err) {
 		xenbus_dev_fatal(dev, err, "writing %s/feature-persistent",
 				 dev->nodename);
@@ -1059,7 +1060,7 @@ static int connect_ring(struct backend_info *be)
 	}
 	pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent",
 					   0);
-	be->blkif->vbd.feature_gnt_persistent = pers_grants;
+	be->blkif->vbd.feature_gnt_persistent = pers_grants && !xen_shim_domain();
 	be->blkif->vbd.overflow_max_grants = 0;
 
 	/*
-- 
2.11.0