[PATCH 2/3] Handle GNTST_eagain in kernel drivers

[Adin Scannell <adin@scannell.ca>]

Handle GNTST_eagain status from GNTTABOP_map_grant_ref and
GNTTABOP_copy operations properly to allow usage of xenpaging without
causing crashes or data corruption.

Replace all relevant HYPERVISOR_grant_table_op() calls with a retry
loop. This loop is implemented as a macro to allow different
GNTTABOP_* args. It will sleep up to 33 seconds and wait for the
page to be paged in again.

All ->status checks were updated to use the GNTST_* namespace. All
return values are converted from GNTST_* namespace to 0/-EINVAL, since
all callers did not use the actual return value.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Patrick Colp <pjcolp@cs.ubc.ca>

This is a port to the mainline Linux tree.  This required dropping many backend
drivers which have not yet made it in.  Additionally, several of the referenced
functions have moved and/or been refactored.  I attempted to minimize changes
while keeping the same semantics.

Signed-off-by: Adin Scannell <adin@scannell.ca>

Index: linux/drivers/block/xen-blkback/blkback.c
===================================================================
---
 drivers/block/xen-blkback/blkback.c |    4 ++-
 drivers/xen/grant-table.c           |    7 ++++-
 drivers/xen/xenbus/xenbus_client.c  |    4 +++
 include/xen/grant_table.h           |   39 +++++++++++++++++++++++++++++++++++
 include/xen/interface/grant_table.h |    6 ++++-
 5 files changed, 56 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 15ec4db..d3fb290 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -390,7 +390,9 @@ static int xen_blkbk_map(struct blkif_request *req,
 	 * the page from the other domain.
 	 */
 	for (i = 0; i < nseg; i++) {
-		if (unlikely(map[i].status != 0)) {
+		if (unlikely(map[i].status == GNTST_eagain))
+			gnttab_check_GNTST_eagain_do_while(GNTTABOP_map_grant_ref, &map[i]);
+		if (unlikely(map[i].status != GNTST_okay)) {
 			pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
 			map[i].handle = BLKBACK_INVALID_HANDLE;
 			ret |= 1;
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index bf1c094..48826c3 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -464,9 +464,12 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
 
 	for (i = 0; i < count; i++) {
 		/* Do not add to override if the map failed. */
-		if (map_ops[i].status)
+		if (map_ops[i].status != GNTST_okay && map_ops[i].status != GNTST_eagain)
 			continue;
 
+		if (map_ops[i].status == GNTST_eagain)
+			return -EAGAIN;
+
 		if (map_ops[i].flags & GNTMAP_contains_pte) {
 			pte = (pte_t *) (mfn_to_virt(PFN_DOWN(map_ops[i].host_addr)) +
 				(map_ops[i].host_addr & ~PAGE_MASK));
@@ -565,7 +568,7 @@ static int gnttab_map(unsigned int start_idx, unsigned int end_idx)
 		return -ENOSYS;
 	}
 
-	BUG_ON(rc || setup.status);
+	BUG_ON(rc || (setup.status != GNTST_okay));
 
 	rc = arch_gnttab_map_shared(frames, nr_gframes, gnttab_max_grant_frames(),
 				    &shared);
diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
index 1906125..d123c78 100644
--- a/drivers/xen/xenbus/xenbus_client.c
+++ b/drivers/xen/xenbus/xenbus_client.c
@@ -455,6 +455,8 @@ int xenbus_map_ring_valloc(struct xenbus_device *dev, int gnt_ref, void **vaddr)
 	if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
 		BUG();
 
+	if (op.status == GNTST_eagain)
+		gnttab_check_GNTST_eagain_do_while(GNTTABOP_map_grant_ref, (&op));
 	if (op.status != GNTST_okay) {
 		free_vm_area(area);
 		xenbus_dev_fatal(dev, op.status,
@@ -499,6 +501,8 @@ int xenbus_map_ring(struct xenbus_device *dev, int gnt_ref,
 	if (HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &op, 1))
 		BUG();
 
+	if (op.status == GNTST_eagain)
+		gnttab_check_GNTST_eagain_do_while(GNTTABOP_map_grant_ref, (&op));
 	if (op.status != GNTST_okay) {
 		xenbus_dev_fatal(dev, op.status,
 				 "mapping in shared page %d from domain %d",
diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
index 11e2dfc..46fac85 100644
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -37,6 +37,8 @@
 #ifndef __ASM_GNTTAB_H__
 #define __ASM_GNTTAB_H__
 
+#include <linux/delay.h>
+
 #include <asm/page.h>
 
 #include <xen/interface/xen.h>
@@ -160,4 +162,41 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
 int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
 		      struct page **pages, unsigned int count);
 
+#define gnttab_check_GNTST_eagain_while(__HCop, __HCarg_p)					\
+do {																		\
+	u8 __hc_delay = 1;														\
+	int __ret;																\
+	while (unlikely((__HCarg_p)->status == GNTST_eagain && __hc_delay)) {	\
+		msleep(__hc_delay++);												\
+		__ret = HYPERVISOR_grant_table_op(__HCop, (__HCarg_p), 1);			\
+		BUG_ON(__ret);														\
+	}																		\
+	if (__hc_delay == 0) {													\
+		printk(KERN_ERR "%s: gnt busy\n", __func__,);						\
+		(__HCarg_p)->status = GNTST_bad_page;								\
+	}																		\
+	if ((__HCarg_p)->status != GNTST_okay)									\
+		printk(KERN_ERR "%s: gnt status %x\n", 								\
+			__func__, (__HCarg_p)->status);									\
+} while(0)
+
+#define gnttab_check_GNTST_eagain_do_while(__HCop, __HCarg_p)			\
+do {																	\
+	u8 __hc_delay = 1;													\
+	int __ret;															\
+	do {																\
+		__ret = HYPERVISOR_grant_table_op(__HCop, (__HCarg_p), 1);		\
+		BUG_ON(__ret);													\
+		if ((__HCarg_p)->status == GNTST_eagain)						\
+			msleep(__hc_delay++);										\
+	} while ((__HCarg_p)->status == GNTST_eagain && __hc_delay);		\
+	if (__hc_delay == 0) {												\
+		printk(KERN_ERR "%s: gnt busy\n", __func__);					\
+		(__HCarg_p)->status = GNTST_bad_page;							\
+	}																	\
+	if ((__HCarg_p)->status != GNTST_okay)								\
+		printk(KERN_ERR "%s: gnt status %x\n", 							\
+			__func__, (__HCarg_p)->status);								\
+} while(0)
+
 #endif /* __ASM_GNTTAB_H__ */
diff --git a/include/xen/interface/grant_table.h b/include/xen/interface/grant_table.h
index 39e5717..ba04080 100644
--- a/include/xen/interface/grant_table.h
+++ b/include/xen/interface/grant_table.h
@@ -363,6 +363,8 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_query_size);
 #define GNTST_permission_denied (-8) /* Not enough privilege for operation.  */
 #define GNTST_bad_page         (-9) /* Specified page was invalid for op.    */
 #define GNTST_bad_copy_arg    (-10) /* copy arguments cross page boundary */
+#define GNTST_address_too_big (-11) /* transfer page address too large.      */
+#define GNTST_eagain          (-12) /* Could not map at the moment. Retry.   */
 
 #define GNTTABOP_error_msgs {                   \
     "okay",                                     \
@@ -375,7 +377,9 @@ DEFINE_GUEST_HANDLE_STRUCT(gnttab_query_size);
     "no spare translation slot in the I/O MMU", \
     "permission denied",                        \
     "bad page",                                 \
-    "copy arguments cross page boundary"        \
+    "copy arguments cross page boundary",       \
+    "page address size too large",              \
+    "could not map at the moment, retry"        \
 }
 
 #endif /* __XEN_PUBLIC_GRANT_TABLE_H__ */
-- 
1.6.2.5