[RFC PATCH 2/2] xen/arm: introduce XENFEAT_grant_map_11

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

The flag specifies that the hypervisor maps a grant page to guest
physical address == machine address of the page in addition to the
normal grant mapping address.

Frontends are allowed to map the same page multiple times using multiple
grant references. On the backend side it can be difficult to find out
the physical address corresponding to a particular machine address,
especially at the completation of a dma operation. To simplify address
translations, we introduce a second mapping of the grant at physical
address == machine address so that dom0 can issue cache maintenance
operations without having to find the pfn.

On ARM enable the flag and this behaviour by default if the domain is
directly mapped.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
---
 xen/common/kernel.c               |    4 ++++
 xen/include/asm-arm/grant_table.h |    3 +--
 xen/include/public/features.h     |    3 +++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 7e83353..556ad14 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -325,6 +325,10 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
                 break;
             }
 #endif
+#ifdef CONFIG_ARM
+            if ( is_domain_direct_mapped(d) )
+                fi.submap |= 1U << XENFEAT_grant_map_11;
+#endif
             break;
         default:
             return -EINVAL;
diff --git a/xen/include/asm-arm/grant_table.h b/xen/include/asm-arm/grant_table.h
index eac8a70..91ba230 100644
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -33,8 +33,7 @@ static inline int replace_grant_supported(void)
     ( ((i >= nr_grant_frames(d->grant_table)) &&                         \
      (i < max_nr_grant_frames)) ? 0 : (d->arch.grant_table_gpfn[i]))
 
-#define gnttab_need_iommu_mapping(d)                    \
-    (is_domain_direct_mapped(d) && need_iommu(d))
+#define gnttab_need_iommu_mapping(d)            (is_domain_direct_mapped(d))
 
 #endif /* __ASM_GRANT_TABLE_H__ */
 /*
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index a149aa6..95b87fd 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -94,6 +94,9 @@
 /* operation as Dom0 is supported */
 #define XENFEAT_dom0                      11
 
+/* Xen also maps grant references at pfn = mfn */
+#define XENFEAT_grant_map_11              12
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
-- 
1.7.10.4