[PATCH 1/7] gnttab: add another compiler barrier to acquire_grant_for_copy()

[Jan Beulich <jbeulich@xxxxxxxx>]

... and mem_sharing_gref_to_gfn().

Like for the fetching of trans_{domid,gref} in the former we ought to also
fetch other fields exactly once.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Strictly speaking mem_sharing_gref_to_gfn() would need another barriers
added on the v2 path, but it being different sub-fields of the flags looks
to make the uses okay there. Thoughts?

Note that the calculation of cache_flags in map_grant_ref() has enough of
a barrier after it already, by virtue of active_entry_release() being
called right afterwards.

The fetching of the frame in gnttab_transfer() is hopefully good enough,
by each fetch happening exactly once. The sole risk there is load tearing,
which can occur elsewhere as well.

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2606,6 +2606,7 @@ acquire_grant_for_copy(
     grant_entry_header_t *shah;
     struct active_grant_entry *act;
     grant_status_t *status;
+    unsigned int shflags;
     uint32_t old_pin;
     domid_t trans_domid;
     grant_ref_t trans_gref;
@@ -2655,8 +2656,11 @@ acquire_grant_for_copy(
         status = &status_entry(rgt, gref);
     }
 
+    shflags = shah->flags;
+    barrier();
+
     old_pin = act->pin;
-    if ( sha2 && (shah->flags & GTF_type_mask) == GTF_transitive )
+    if ( sha2 && (shflags & GTF_type_mask) == GTF_transitive )
     {
         if ( (!old_pin || (!readonly &&
                            !(old_pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask)))) 
&&
@@ -2793,6 +2797,8 @@ acquire_grant_for_copy(
     else if ( !old_pin ||
               (!readonly && !(old_pin & (GNTPIN_devw_mask|GNTPIN_hstw_mask))) )
     {
+        unsigned long gfn;
+
         if ( (rc = _set_status(shah, status, rd, rgt->gt_version, act,
                                readonly, 0, ldom)) != GNTST_okay )
              goto unlock_out;
@@ -2801,39 +2807,34 @@ acquire_grant_for_copy(
         trans_gref = gref;
         if ( !sha2 )
         {
-            unsigned long gfn = shared_entry_v1(rgt, gref).frame;
-
-            rc = get_paged_frame(gfn, &grant_mfn, page, readonly, rd);
-            if ( rc != GNTST_okay )
-                goto unlock_out_clear;
-            act_set_gfn(act, _gfn(gfn));
+            gfn = shared_entry_v1(rgt, gref).frame;
             is_sub_page = false;
             trans_page_off = 0;
             trans_length = PAGE_SIZE;
         }
-        else if ( !(sha2->hdr.flags & GTF_sub_page) )
+        else if ( !(shflags & GTF_sub_page) )
         {
-            rc = get_paged_frame(sha2->full_page.frame, &grant_mfn, page,
-                                 readonly, rd);
-            if ( rc != GNTST_okay )
-                goto unlock_out_clear;
-            act_set_gfn(act, _gfn(sha2->full_page.frame));
+            gfn = sha2->full_page.frame;
             is_sub_page = false;
             trans_page_off = 0;
             trans_length = PAGE_SIZE;
         }
         else
         {
-            rc = get_paged_frame(sha2->sub_page.frame, &grant_mfn, page,
-                                 readonly, rd);
-            if ( rc != GNTST_okay )
-                goto unlock_out_clear;
-            act_set_gfn(act, _gfn(sha2->sub_page.frame));
+            gfn = sha2->sub_page.frame;
             is_sub_page = true;
             trans_page_off = sha2->sub_page.page_off;
             trans_length = sha2->sub_page.length;
         }
 
+        /* Use latched copies of the shared entry fields from here on. */
+        barrier();
+
+        rc = get_paged_frame(gfn, &grant_mfn, page, readonly, rd);
+        if ( rc != GNTST_okay )
+            goto unlock_out_clear;
+        act_set_gfn(act, _gfn(gfn));
+
         if ( !act->pin )
         {
             act->domid = ldom;
@@ -4145,6 +4146,8 @@ int mem_sharing_gref_to_gfn(struct grant
            *gfn = _gfn(sha2->full_page.frame);
     }
 
+    barrier();
+
     if ( !rc && (flags & GTF_type_mask) != GTF_permit_access )
         rc = -ENXIO;
     else if ( !rc && status )