[PATCH v3 12/21] xen/xenbus: add xenbus_setup_ring() service function

Juergen Gross
2022-05-05T08:18:06Z

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@suse.com>
---
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);

--
2.35.3