[Xen-devel] [PATCH 02 of 13] x86/mm: Update mem sharing interface to (re)allow sharing of grants

[Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>]

 xen/arch/x86/mm/mem_sharing.c |  57 ++++++++++++++++++++++++++++++++++++------
 xen/include/public/domctl.h   |  13 +++++++++
 2 files changed, 61 insertions(+), 9 deletions(-)


Previosuly, the mem sharing code would return an opaque handle to index shared
pages (and nominees) in its global hash table.  By removing the hash table, the
new interfaces requires a gfn and a version. However, when sharing grants, the
caller provides a grant ref and a version. Update interface to handle this
case.

The use case for grant sharing is when sharing from within a backend (e.g.
memshr + blktap2), in which case the backend is only exposed to grant
references.

Signed-off-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
Signed-off-by: Adin Scannell <adin@xxxxxxxxxxx>

diff -r bcc4215663e8 -r 61e164d6e956 xen/arch/x86/mm/mem_sharing.c
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -728,18 +728,57 @@ int mem_sharing_domctl(struct domain *d,
 
         case XEN_DOMCTL_MEM_EVENT_OP_SHARING_SHARE:
         {
-            unsigned long sgfn  = mec->u.share.source_gfn;
-            shr_handle_t sh     = mec->u.share.source_handle;
-            struct domain *cd   = get_domain_by_id(mec->u.share.client_domain);
-            if ( cd )
+            unsigned long sgfn, cgfn;
+            struct domain *cd;
+            shr_handle_t sh, ch;
+
+            if ( !mem_sharing_enabled(d) )
+                return -EINVAL;
+
+            cd = get_domain_by_id(mec->u.share.client_domain);
+            if ( !cd )
+                return -ESRCH;
+
+            if ( !mem_sharing_enabled(cd) )
             {
-                unsigned long cgfn  = mec->u.share.client_gfn;
-                shr_handle_t ch     = mec->u.share.client_handle;
-                rc = mem_sharing_share_pages(d, sgfn, sh, cd, cgfn, ch); 
                 put_domain(cd);
+                return -EINVAL;
             }
-            else
-                return -EEXIST;
+
+            if ( XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF(mec->u.share.source_gfn) 
)
+            {
+                grant_ref_t gref = (grant_ref_t) 
+                                    (XEN_DOMCTL_MEM_SHARING_FIELD_GET_GREF(
+                                        mec->u.share.source_gfn));
+                if ( mem_sharing_gref_to_gfn(d, gref, &sgfn) < 0 )
+                {
+                    put_domain(cd);
+                    return -EINVAL;
+                }
+            } else {
+                sgfn  = mec->u.share.source_gfn;
+            }
+
+            if ( XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF(mec->u.share.client_gfn) 
)
+            {
+                grant_ref_t gref = (grant_ref_t) 
+                                    (XEN_DOMCTL_MEM_SHARING_FIELD_GET_GREF(
+                                        mec->u.share.client_gfn));
+                if ( mem_sharing_gref_to_gfn(cd, gref, &cgfn) < 0 )
+                {
+                    put_domain(cd);
+                    return -EINVAL;
+                }
+            } else {
+                cgfn  = mec->u.share.client_gfn;
+            }
+
+            sh = mec->u.share.source_handle;
+            ch = mec->u.share.client_handle;
+
+            rc = mem_sharing_share_pages(d, sgfn, sh, cd, cgfn, ch); 
+
+            put_domain(cd);
         }
         break;
 
diff -r bcc4215663e8 -r 61e164d6e956 xen/include/public/domctl.h
--- a/xen/include/public/domctl.h
+++ b/xen/include/public/domctl.h
@@ -775,6 +775,19 @@ DEFINE_XEN_GUEST_HANDLE(xen_domctl_mem_e
 #define XEN_DOMCTL_MEM_SHARING_S_HANDLE_INVALID  (-10)
 #define XEN_DOMCTL_MEM_SHARING_C_HANDLE_INVALID  (-9)
 
+/* The following allows sharing of grant refs. This is useful
+ * for sharing utilities sitting as "filters" in IO backends
+ * (e.g. memshr + blktap(2)). The IO backend is only exposed 
+ * to grant references, and this allows sharing of the grefs */
+#define XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF_FLAG   (1ULL << 62)
+
+#define XEN_DOMCTL_MEM_SHARING_FIELD_MAKE_GREF(field, val)  \
+    (field) = (XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF_FLAG | val)
+#define XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF(field)         \
+    ((field) & XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF_FLAG)
+#define XEN_DOMCTL_MEM_SHARING_FIELD_GET_GREF(field)        \
+    ((field) & (~XEN_DOMCTL_MEM_SHARING_FIELD_IS_GREF_FLAG))
+
 struct xen_domctl_mem_sharing_op {
     uint8_t op; /* XEN_DOMCTL_MEM_EVENT_OP_* */
 

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