[PATCH v3 3/3] xen/arm: introduce XENFEAT_grant_map_identity

[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.

Call arch_grant_map_page_identity and arch_grant_unmap_page_identity
from __gnttab_map_grant_ref and __gnttab_unmap_common to introduce the
second mapping if the domain is directly mapped.

Introduce gnttab_need_identity_mapping, to check if a domain needs the
identity mapping. On x86 it just returns always 0.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

---

Changes in v3:
- introduce gnttab_need_identity_mapping;
- check gnttab_need_identity_mapping in __gnttab_map_grant_ref and
__gnttab_unmap_common.

Changes in v2:
- rename XENFEAT_grant_map_11 to XENFEAT_grant_map_identity;
- remove superfluous ifdef CONFIG_ARM in xen/common/kernel.c;
- don't modify gnttab_need_iommu_mapping;
- call arch_grant_map_page_identity and arch_grant_unmap_page_identity
from grant_table functions.
---
 xen/common/grant_table.c          |   35 +++++++++++++++++++++++++++--------
 xen/common/kernel.c               |    3 +++
 xen/include/asm-arm/grant_table.h |    3 +++
 xen/include/asm-x86/grant_table.h |    2 ++
 xen/include/public/features.h     |    3 +++
 5 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 464007e..67cbed9 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -727,7 +727,7 @@ __gnttab_map_grant_ref(
 
     double_gt_lock(lgt, rgt);
 
-    if ( gnttab_need_iommu_mapping(ld) )
+    if ( gnttab_need_iommu_mapping(ld) || gnttab_need_identity_mapping(ld) )
     {
         unsigned int wrc, rdc;
         int err = 0;
@@ -738,13 +738,23 @@ __gnttab_map_grant_ref(
              !(old_pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask)) )
         {
             if ( wrc == 0 )
-                err = iommu_map_page(ld, frame, frame,
-                                     IOMMUF_readable|IOMMUF_writable);
+            {
+                if ( gnttab_need_identity_mapping(ld) )
+                    err = arch_grant_map_page_identity(ld, frame, 1);
+                else
+                    err = iommu_map_page(ld, frame, frame,
+                            IOMMUF_readable|IOMMUF_writable);
+            }
         }
         else if ( act_pin && !old_pin )
         {
             if ( (wrc + rdc) == 0 )
-                err = iommu_map_page(ld, frame, frame, IOMMUF_readable);
+            {
+                if ( gnttab_need_identity_mapping(ld) )
+                    err = arch_grant_map_page_identity(ld, frame, 0);
+                else
+                    err = iommu_map_page(ld, frame, frame, IOMMUF_readable);
+            }
         }
         if ( err )
         {
@@ -935,15 +945,24 @@ __gnttab_unmap_common(
             act->pin -= GNTPIN_hstw_inc;
     }
 
-    if ( gnttab_need_iommu_mapping(ld) )
+    if ( gnttab_need_iommu_mapping(ld) || gnttab_need_identity_mapping(ld) )
     {
         unsigned int wrc, rdc;
         int err = 0;
         mapcount(lgt, rd, op->frame, &wrc, &rdc);
         if ( (wrc + rdc) == 0 )
-            err = iommu_unmap_page(ld, op->frame);
-        else if ( wrc == 0 )
-            err = iommu_map_page(ld, op->frame, op->frame, IOMMUF_readable);
+        {
+            if ( gnttab_need_identity_mapping(ld) )
+                err = arch_grant_unmap_page_identity(ld, op->frame);
+            else
+                err = iommu_unmap_page(ld, op->frame);
+        } else if ( wrc == 0 )
+        {
+            if ( gnttab_need_identity_mapping(ld) )
+                err = arch_grant_map_page_identity(ld, op->frame, 0);
+            else
+                err = iommu_map_page(ld, op->frame, op->frame, IOMMUF_readable);
+        }
         if ( err )
         {
             rc = GNTST_general_error;
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 7e83353..dacbe38 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -15,6 +15,7 @@
 #include <xen/guest_access.h>
 #include <xen/hypercall.h>
 #include <asm/current.h>
+#include <asm/grant_table.h>
 #include <public/nmi.h>
 #include <public/version.h>
 
@@ -325,6 +326,8 @@ DO(xen_version)(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
                 break;
             }
 #endif
+            if ( gnttab_need_identity_mapping(d) )
+                fi.submap |= 1U << XENFEAT_grant_map_identity;
             break;
         default:
             return -EINVAL;
diff --git a/xen/include/asm-arm/grant_table.h b/xen/include/asm-arm/grant_table.h
index eac8a70..6f7ccd9 100644
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -36,6 +36,9 @@ static inline int replace_grant_supported(void)
 #define gnttab_need_iommu_mapping(d)                    \
     (is_domain_direct_mapped(d) && need_iommu(d))
 
+#define gnttab_need_identity_mapping(d)                    \
+    (is_domain_direct_mapped(d) && !need_iommu(d))
+
 #endif /* __ASM_GRANT_TABLE_H__ */
 /*
  * Local variables:
diff --git a/xen/include/asm-x86/grant_table.h b/xen/include/asm-x86/grant_table.h
index 8c9bbcf..56861a7 100644
--- a/xen/include/asm-x86/grant_table.h
+++ b/xen/include/asm-x86/grant_table.h
@@ -68,6 +68,8 @@ static inline void gnttab_clear_flag(unsigned int nr, uint16_t *st)
 #define gnttab_need_iommu_mapping(d)                \
     (!paging_mode_translate(d) && need_iommu(d))
 
+#define gnttab_need_identity_mapping(d)     (0)
+
 static inline int replace_grant_supported(void)
 {
     return 1;
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index a149aa6..ba753a0 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_identity              12
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */
-- 
1.7.10.4