gnttab: make sure grant map operations don't skip their IOMMU part

Two almost simultaneous mapping requests need to make sure that at the
completion of the earlier one IOMMU mappings (established explicitly
here in the PV case) have been put in place. Forever since the splitting
of the grant table lock a violation of this has been possible (using
simplified pin counts, as it doesn't matter whether we talk about read
or write mappings here):

initial state: act->pin = 0

vCPU A: progress the operation past the dropping of the locks after the
        act->pin updates (act->pin = 1, old_pin = 0, act_pin = 1)

vCPU B: progress the operation past the dropping of the locks after the
        act->pin updates (act->pin = 2, old_pin = 1, act_pin = 2)

vCPU B: (re-)acquire both gt locks, mapkind() returns 0, but both
        iommu_legacy_map() invocations get skipped due to non-zero
        old_pin

vCPU B: return to caller without IOMMU mapping

vCPU A: (re-)acquire both gt locks, mapkind() returns 0,
        iommu_legacy_map() gets invoked

With the locks dropped intermediately, whether to invoke
iommu_legacy_map() must depend on only the return value of mapkind()
and of course the kind of mapping request being processed, just like
is already the case in unmap_common().

Also fix the style of the adjacent comment, and correct a nearby one
still referring to a prior name of what is now mapkind().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 921f1f4226
master date: 2019-12-03 14:13:40 +0100

xen/common/grant_table.c

@@ -919,8 +919,6 @@ map_grant_ref(
     mfn_t frame;
     struct page_info *pg = NULL;
     int            rc = GNTST_okay;
-    u32            old_pin;
-    u32            act_pin;
     unsigned int   cache_flags, refcnt = 0, typecnt = 0;
     bool           host_map_created = false;
     struct active_grant_entry *act = NULL;
@@ -1024,7 +1022,6 @@ map_grant_ref(
         }
     }
 
-    old_pin = act->pin;
     if ( op->flags & GNTMAP_device_map )
         act->pin += (op->flags & GNTMAP_readonly) ?
             GNTPIN_devr_inc : GNTPIN_devw_inc;
@@ -1033,7 +1030,6 @@ map_grant_ref(
             GNTPIN_hstr_inc : GNTPIN_hstw_inc;
 
     frame = act->frame;
-    act_pin = act->pin;
 
     cache_flags = (shah->flags & (GTF_PAT | GTF_PWT | GTF_PCD) );
 
@@ -1141,27 +1137,22 @@ map_grant_ref(
     if ( need_iommu )
     {
         unsigned int kind;
-        int err = 0;
 
         double_gt_lock(lgt, rgt);
 
-        /* We're not translated, so we know that gmfns and mfns are
-           the same things, so the IOMMU entry is always 1-to-1. */
+        /*
+         * We're not translated, so we know that dfns and mfns are
+         * the same things, so the IOMMU entry is always 1-to-1.
+         */
         kind = mapkind(lgt, rd, frame);
-        if ( (act_pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask)) &&
-             !(old_pin & (GNTPIN_hstw_mask|GNTPIN_devw_mask)) )
-        {
-            if ( !(kind & MAPKIND_WRITE) )
-                err = iommu_map_page(ld, mfn_x(frame), mfn_x(frame),
-                                     IOMMUF_readable|IOMMUF_writable);
-        }
-        else if ( act_pin && !old_pin )
-        {
-            if ( !kind )
-                err = iommu_map_page(ld, mfn_x(frame), mfn_x(frame),
-                                     IOMMUF_readable);
-        }
-        if ( err )
+        if ( !(op->flags & GNTMAP_readonly) &&
+             !(kind & MAPKIND_WRITE) )
+            kind = IOMMUF_readable | IOMMUF_writable;
+        else if ( !kind )
+            kind = IOMMUF_readable;
+        else
+            kind = 0;
+        if ( kind && iommu_map_page(ld, mfn_x(frame), mfn_x(frame), kind) )
         {
             double_gt_unlock(lgt, rgt);
             rc = GNTST_general_error;
@@ -1176,7 +1167,7 @@ map_grant_ref(
      * other fields so just ensure the flags field is stored last.
      *
      * However, if gnttab_need_iommu_mapping() then this would race
-     * with a concurrent mapcount() call (on an unmap, for example)
+     * with a concurrent mapkind() call (on an unmap, for example)
      * and a lock is required.
      */
     mt = &maptrack_entry(lgt, handle);