[Xen-devel] [PATCH v14 02/17] libxc: Move temporary grant table mapping to end of memory

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

From: Roger Pau Monnà <roger.pau@xxxxxxxxxx>

In order to set up the grant table for HVM guests, libxc needs to map
the grant table temporarily.  At the moment, it does this by adding the
grant page to the HVM guest's p2m table in the MMIO hole (at gfn 0xFFFFE),
then mapping that gfn, setting up the table, then unmapping the gfn and
removing it from the p2m table.

This breaks with PVH guests with 4G or more of ram, because there is
no MMIO hole; so it ends up clobbering a valid RAM p2m entry, then
leaving a "hole" when it removes the grant map from the p2m table.
Since the guest thinks this is normal ram, when it maps it and tries
to access the page, it crashes.

This patch maps the page at max_gfn+1 instead.

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
---
 tools/libxc/xc_dom.h      |    3 ---
 tools/libxc/xc_dom_boot.c |   14 ++++++++++++--
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/tools/libxc/xc_dom.h b/tools/libxc/xc_dom.h
index 86e23ee..935b49e 100644
--- a/tools/libxc/xc_dom.h
+++ b/tools/libxc/xc_dom.h
@@ -18,9 +18,6 @@
 
 #define INVALID_P2M_ENTRY   ((xen_pfn_t)-1)
 
-/* Scrach PFN for temporary mappings in HVM */
-#define SCRATCH_PFN_GNTTAB 0xFFFFE
-
 /* --- typedefs and structs ---------------------------------------- */
 
 typedef uint64_t xen_vaddr_t;
diff --git a/tools/libxc/xc_dom_boot.c b/tools/libxc/xc_dom_boot.c
index 71e1897..fdfeaf8 100644
--- a/tools/libxc/xc_dom_boot.c
+++ b/tools/libxc/xc_dom_boot.c
@@ -361,17 +361,27 @@ int xc_dom_gnttab_hvm_seed(xc_interface *xch, domid_t 
domid,
                            domid_t xenstore_domid)
 {
     int rc;
+    xen_pfn_t max_gfn;
     struct xen_add_to_physmap xatp = {
         .domid = domid,
         .space = XENMAPSPACE_grant_table,
         .idx   = 0,
-        .gpfn  = SCRATCH_PFN_GNTTAB
     };
     struct xen_remove_from_physmap xrfp = {
         .domid = domid,
-        .gpfn  = SCRATCH_PFN_GNTTAB
     };
 
+    max_gfn = xc_domain_maximum_gpfn(xch, domid);
+    if ( max_gfn <= 0 ) {
+        xc_dom_panic(xch, XC_INTERNAL_ERROR,
+                     "%s: failed to get max gfn "
+                     "[errno=%d]\n",
+                     __FUNCTION__, errno);
+        return -1;
+    }
+    xatp.gpfn = max_gfn + 1;
+    xrfp.gpfn = max_gfn + 1;
+
     rc = do_memory_op(xch, XENMEM_add_to_physmap, &xatp, sizeof(xatp));
     if ( rc != 0 )
     {
-- 
1.7.9.5


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