[PATCH] xen-blkback: only attach blkback if the required features are met

[Roger Pau Monne <roger.pau@citrix.com>]

Blkback cannot work properly on auto-translated guests if Xen doesn't
update the IOMMU when performing grant maps/unmaps, so only attach if
the newly introduced XENFEAT_hvm_gntmap_supports_iommu is found.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
---
 drivers/block/xen-blkback/blkback.c |   12 ++++++++++++
 include/xen/interface/features.h    |    6 ++++++
 2 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 64c60ed..4c23ddd 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -1381,6 +1381,18 @@ static int __init xen_blkif_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+#ifdef CONFIG_X86
+	if (xen_feature(XENFEAT_auto_translated_physmap) &&
+	    !xen_feature(XENFEAT_hvm_gntmap_supports_iommu)) {
+		/*
+		 * blkback cannot work properly on auto-translated guests
+		 * if grant table mappings don't update the IOMMU entries.
+		 */
+		pr_debug(DRV_PFX "Disabling blkback because Xen is missing feature XENFEAT_hvm_gntmap_supports_iommu\n");
+		return -ENODEV;
+	}
+#endif /* CONFIG_X86 */
+
 	rc = xen_blkif_interface_init();
 	if (rc)
 		goto failed_init;
diff --git a/include/xen/interface/features.h b/include/xen/interface/features.h
index 131a6cc..d1e0f23 100644
--- a/include/xen/interface/features.h
+++ b/include/xen/interface/features.h
@@ -53,6 +53,12 @@
 /* operation as Dom0 is supported */
 #define XENFEAT_dom0                      11
 
+/*
+ * If set, GNTTABOP_map_grant_ref sets the proper IOMMU mappings so the
+ * resulting mapped page can be used for IO with hardware devices.
+ */
+#define XENFEAT_hvm_gntmap_supports_iommu 12
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
-- 
1.7.7.5 (Apple Git-26)