[PATCH v2 10/19] xen/xenbus: add xenbus_setup_ring() service function

[Juergen Gross <jgross@xxxxxxxx>]

Most PV device frontends share very similar code for setting up shared
ring buffers:

- allocate page(s)
- init the ring admin data
- give the backend access to the ring via grants

Tearing down the ring requires similar actions in all frontends again:

- remove grants
- free the page(s)

Provide service functions xenbus_setup_ring() and xenbus_teardown_ring()
for that purpose.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
 drivers/xen/xenbus/xenbus_client.c | 69 ++++++++++++++++++++++++++++++
 include/xen/xenbus.h               |  4 ++
 2 files changed, 73 insertions(+)

diff --git a/drivers/xen/xenbus/xenbus_client.c 
b/drivers/xen/xenbus/xenbus_client.c
index df6890681231..1a2e0d94ccd1 100644
--- a/drivers/xen/xenbus/xenbus_client.c
+++ b/drivers/xen/xenbus/xenbus_client.c
@@ -407,6 +407,75 @@ int xenbus_grant_ring(struct xenbus_device *dev, void 
*vaddr,
 }
 EXPORT_SYMBOL_GPL(xenbus_grant_ring);
 
+/*
+ * xenbus_setup_ring
+ * @dev: xenbus device
+ * @vaddr: pointer to starting virtual address of the ring
+ * @nr_pages: number of pages to be granted
+ * @grefs: grant reference array to be filled in
+ *
+ * Allocate physically contiguous pages for a shared ring buffer and grant it
+ * to the peer of the given device. The ring buffer is initially filled with
+ * zeroes. The virtual address of the ring is stored at @vaddr and the
+ * grant references are stored in the @grefs array. In case of error @vaddr
+ * will be set to NULL and @grefs will be filled with INVALID_GRANT_REF.
+ */
+int xenbus_setup_ring(struct xenbus_device *dev, gfp_t gfp, void **vaddr,
+                     unsigned int nr_pages, grant_ref_t *grefs)
+{
+       unsigned long ring_size = nr_pages * XEN_PAGE_SIZE;
+       unsigned int i;
+       int ret;
+
+       *vaddr = alloc_pages_exact(ring_size, gfp | __GFP_ZERO);
+       if (!*vaddr) {
+               ret = -ENOMEM;
+               goto err;
+       }
+
+       ret = xenbus_grant_ring(dev, *vaddr, nr_pages, grefs);
+       if (ret)
+               goto err;
+
+       return 0;
+
+ err:
+       if (*vaddr)
+               free_pages_exact(*vaddr, ring_size);
+       for (i = 0; i < nr_pages; i++)
+               grefs[i] = INVALID_GRANT_REF;
+       *vaddr = NULL;
+
+       return ret;
+}
+EXPORT_SYMBOL_GPL(xenbus_setup_ring);
+
+/*
+ * xenbus_teardown_ring
+ * @vaddr: starting virtual address of the ring
+ * @nr_pages: number of pages
+ * @grefs: grant reference array
+ *
+ * Remove grants for the shared ring buffer and free the associated memory.
+ * On return the grant reference array is filled with INVALID_GRANT_REF.
+ */
+void xenbus_teardown_ring(void **vaddr, unsigned int nr_pages,
+                         grant_ref_t *grefs)
+{
+       unsigned int i;
+
+       for (i = 0; i < nr_pages; i++) {
+               if (grefs[i] != INVALID_GRANT_REF) {
+                       gnttab_end_foreign_access(grefs[i], 0);
+                       grefs[i] = INVALID_GRANT_REF;
+               }
+       }
+
+       if (*vaddr)
+               free_pages_exact(*vaddr, nr_pages * XEN_PAGE_SIZE);
+       *vaddr = NULL;
+}
+EXPORT_SYMBOL_GPL(xenbus_teardown_ring);
 
 /**
  * Allocate an event channel for the given xenbus_device, assigning the newly
diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h
index b13eb86395e0..b533b4adc835 100644
--- a/include/xen/xenbus.h
+++ b/include/xen/xenbus.h
@@ -226,6 +226,10 @@ int xenbus_watch_pathfmt(struct xenbus_device *dev, struct 
xenbus_watch *watch,
 int xenbus_switch_state(struct xenbus_device *dev, enum xenbus_state 
new_state);
 int xenbus_grant_ring(struct xenbus_device *dev, void *vaddr,
                      unsigned int nr_pages, grant_ref_t *grefs);
+int xenbus_setup_ring(struct xenbus_device *dev, gfp_t gfp, void **vaddr,
+                     unsigned int nr_pages, grant_ref_t *grefs);
+void xenbus_teardown_ring(void **vaddr, unsigned int nr_pages,
+                         grant_ref_t *grefs);
 int xenbus_map_ring_valloc(struct xenbus_device *dev, grant_ref_t *gnt_refs,
                           unsigned int nr_grefs, void **vaddr);
 
-- 
2.34.1