[Xen-devel] [PATCH v3] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Or if it's not possible to honor the hinted address an error is returned
instead. This makes it easier to spot the actual failure, instead of
failing later on when the caller of xen_remap_bucket realizes the
mapping has not been created at the requested address.

Also note that at least on FreeBSD using MAP_FIXED will cause mmap to
try harder to honor the passed address.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
---
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Anthony Perard <anthony.perard@xxxxxxxxxx>
Cc: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Richard Henderson <rth@xxxxxxxxxxx>
Cc: Eduardo Habkost <ehabkost@xxxxxxxxxx>
Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
Cc: Marcel Apfelbaum <marcel.apfelbaum@xxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
---
Changes since v2:
 - Move comment position.

Changes since v1:
 - Use MAP_FIXED for the dummy mmap call also if a specific virtual
   address is requested.
---
 hw/i386/xen/xen-mapcache.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index 349f72d00c..254759f776 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -184,9 +184,14 @@ static void xen_remap_bucket(MapCacheEntry *entry,
         pfns[i] = (address_index << (MCACHE_BUCKET_SHIFT-XC_PAGE_SHIFT)) + i;
     }
 
+    /*
+     * If the caller has requested the mapping at a specific address use
+     * MAP_FIXED to make sure it's honored.
+     */
     if (!dummy) {
         vaddr_base = xenforeignmemory_map2(xen_fmem, xen_domid, vaddr,
-                                           PROT_READ | PROT_WRITE, 0,
+                                           PROT_READ | PROT_WRITE,
+                                           vaddr ? MAP_FIXED : 0,
                                            nb_pfn, pfns, err);
         if (vaddr_base == NULL) {
             perror("xenforeignmemory_map2");
@@ -198,7 +203,8 @@ static void xen_remap_bucket(MapCacheEntry *entry,
          * mapping immediately due to certain circumstances (i.e. on resume 
now)
          */
         vaddr_base = mmap(vaddr, size, PROT_READ | PROT_WRITE,
-                          MAP_ANON | MAP_SHARED, -1, 0);
+                          MAP_ANON | MAP_SHARED | (vaddr ? MAP_FIXED : 0),
+                          -1, 0);
         if (vaddr_base == MAP_FAILED) {
             perror("mmap");
             exit(-1);
-- 
2.17.2 (Apple Git-113)


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