xen: Preserve reserved grant entries when switching versions

In order for the toolstack to use reserved grant table entries, the
grant table for a guest must be initialized prior to the guest's boot.
When the guest switches grant table versions (necessary if the guest
is using v2 grant tables, or on kexec if switching grant versions),
these initial grants will be cleared. Instead of clearing them,
preserve the grants across the type change.

Attempting to preserve v2-only features such as sub-page grants will
produce a warning and clear the resulting v1 grant entry.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>

2 files changed, 57 insertions, 8 deletions

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 0c55fd154d..858d991d3d 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2111,6 +2111,7 @@ gnttab_set_version(XEN_GUEST_HANDLE(gnttab_set_version_t uop))
     struct domain *d = current->domain;
     struct grant_table *gt = d->grant_table;
     struct active_grant_entry *act;
+    grant_entry_v1_t reserved_entries[GNTTAB_NR_RESERVED_ENTRIES];
     long res;
     int i;
 
@@ -2127,11 +2128,12 @@ gnttab_set_version(XEN_GUEST_HANDLE(gnttab_set_version_t uop))
 
     spin_lock(&gt->lock);
     /* Make sure that the grant table isn't currently in use when we
-       change the version number. */
-    /* (You need to change the version number for e.g. kexec.) */
+       change the version number, except for the first 8 entries which
+       are allowed to be in use (xenstore/xenconsole keeps them mapped).
+       (You need to change the version number for e.g. kexec.) */
     if ( gt->gt_version != 0 )
     {
-        for ( i = 0; i < nr_grant_entries(gt); i++ )
+        for ( i = GNTTAB_NR_RESERVED_ENTRIES; i < nr_grant_entries(gt); i++ )
         {
             act = &active_entry(gt, i);
             if ( act->pin != 0 )
@@ -2156,15 +2158,55 @@ gnttab_set_version(XEN_GUEST_HANDLE(gnttab_set_version_t uop))
             goto out_unlock;
     }
 
+    /* Preserve the first 8 entries (toolstack reserved grants) */
+    if ( gt->gt_version == 1 )
+    {
+        memcpy(reserved_entries, &shared_entry_v1(gt, 0), sizeof(reserved_entries));
+    }
+    else if ( gt->gt_version == 2 )
+    {
+        for ( i = 0; i < GNTTAB_NR_RESERVED_ENTRIES && i < nr_grant_entries(gt); i++ )
+        {
+            int flags = status_entry(gt, i);
+            flags |= shared_entry_v2(gt, i).hdr.flags;
+            if ((flags & GTF_type_mask) == GTF_permit_access)
+            {
+                reserved_entries[i].flags = flags;
+                reserved_entries[i].domid = shared_entry_v2(gt, i).hdr.domid;
+                reserved_entries[i].frame = shared_entry_v2(gt, i).full_page.frame;
+            }
+            else
+            {
+                if ((flags & GTF_type_mask) != GTF_invalid)
+                    gdprintk(XENLOG_INFO, "d%d: bad flags %x in grant %d when switching grant version\n",
+                           d->domain_id, flags, i);
+                memset(&reserved_entries[i], 0, sizeof(reserved_entries[i]));
+            }
+        }
+    }
+
     if ( op.version < 2 && gt->gt_version == 2 )
         gnttab_unpopulate_status_frames(d, gt);
 
-    if ( op.version != gt->gt_version )
+    /* Make sure there's no crud left over in the table from the
+       old version. */
+    for ( i = 0; i < nr_grant_frames(gt); i++ )
+        memset(gt->shared_raw[i], 0, PAGE_SIZE);
+
+    /* Restore the first 8 entries (toolstack reserved grants) */
+    if ( gt->gt_version != 0 && op.version == 1 )
     {
-        /* Make sure there's no crud left over in the table from the
-           old version. */
-        for ( i = 0; i < nr_grant_frames(gt); i++ )
-            memset(gt->shared_raw[i], 0, PAGE_SIZE);
+        memcpy(&shared_entry_v1(gt, 0), reserved_entries, sizeof(reserved_entries));
+    }
+    else if ( gt->gt_version != 0 && op.version == 2 )
+    {
+        for ( i = 0; i < GNTTAB_NR_RESERVED_ENTRIES; i++ )
+        {
+            status_entry(gt, i) = reserved_entries[i].flags & (GTF_reading|GTF_writing);
+            shared_entry_v2(gt, i).hdr.flags = reserved_entries[i].flags & ~(GTF_reading|GTF_writing);
+            shared_entry_v2(gt, i).hdr.domid = reserved_entries[i].domid;
+            shared_entry_v2(gt, i).full_page.frame = reserved_entries[i].frame;
+        }
     }
 
     gt->gt_version = op.version;
diff --git a/xen/include/public/grant_table.h b/xen/include/public/grant_table.h
index 54d95511e9..292d7243ec 100644
--- a/xen/include/public/grant_table.h
+++ b/xen/include/public/grant_table.h
@@ -117,6 +117,13 @@ struct grant_entry_v1 {
 };
 typedef struct grant_entry_v1 grant_entry_v1_t;
 
+/* The first few grant table entries will be preserved across grant table
+ * version changes and may be pre-populated at domain creation by tools.
+ */
+#define GNTTAB_NR_RESERVED_ENTRIES     8
+#define GNTTAB_RESERVED_CONSOLE        0
+#define GNTTAB_RESERVED_XENSTORE       1
+
 /*
  * Type of grant entry.
  *  GTF_invalid: This grant entry grants no privileges.