migration/postcopy: Explicitly disallow huge pages

At the moment postcopy will fail as soon as qemu tries to register
userfault on the RAMBlock pages that are backed by hugepages.
However, the kernel is going to get userfault support for hugepage
at some point, and we've not got the rest of the QEMU code to support
it yet, so fail neatly with an error like:

Postcopy doesn't support hugetlbfs yet (/objects/mem1)

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/postcopy-ram.c

@@ -84,6 +84,24 @@ static bool ufd_version_check(int ufd)
     return true;
 }
 
+/*
+ * Check for things that postcopy won't support; returns 0 if the block
+ * is fine.
+ */
+static int check_range(const char *block_name, void *host_addr,
+                      ram_addr_t offset, ram_addr_t length, void *opaque)
+{
+    RAMBlock *rb = qemu_ram_block_by_name(block_name);
+
+    if (qemu_ram_pagesize(rb) > getpagesize()) {
+        error_report("Postcopy doesn't support large page sizes yet (%s)",
+                     block_name);
+        return -E2BIG;
+    }
+
+    return 0;
+}
+
 /*
  * Note: This has the side effect of munlock'ing all of RAM, that's
  * normally fine since if the postcopy succeeds it gets turned back on at the
@@ -104,6 +122,12 @@ bool postcopy_ram_supported_by_host(void)
         goto out;
     }
 
+    /* Check for anything about the RAMBlocks we don't support */
+    if (qemu_ram_foreach_block(check_range, NULL)) {
+        /* check_range will have printed its own error */
+        goto out;
+    }
+
     ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
     if (ufd == -1) {
         error_report("%s: userfaultfd not available: %s", __func__,