hostmem-file: add "align" option

When mmap(2) the backend files, QEMU uses the host page size
(getpagesize(2)) by default as the alignment of mapping address.
However, some backends may require alignments different than the page
size. For example, mmap a device DAX (e.g., /dev/dax0.0) on Linux
kernel 4.13 to an address, which is 4K-aligned but not 2M-aligned,
fails with a kernel message like

[617494.969768] dax dax0.0: qemu-system-x86: dax_mmap: fail, unaligned vma (0x7fa37c579000 - 0x7fa43c579000, 0x1fffff)

Because there is no common approach to get such alignment requirement,
we add the 'align' option to 'memory-backend-file', so that users or
management utils, which have enough knowledge about the backend, can
specify a proper alignment via this option.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20171211072806.2812-2-haozhong.zhang@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[ehabkost: fixed typo, fixed error_setg() format string]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

1 files changed, 40 insertions, 1 deletions

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index e44c319..e319ec1 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -34,6 +34,7 @@ struct HostMemoryBackendFile {
     bool share;
     bool discard_data;
     char *mem_path;
+    uint64_t align;
 };
 
 static void
@@ -58,7 +59,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         path = object_get_canonical_path(OBJECT(backend));
         memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
                                  path,
-                                 backend->size, fb->share,
+                                 backend->size, fb->align, fb->share,
                                  fb->mem_path, errp);
         g_free(path);
     }
@@ -115,6 +116,40 @@ static void file_memory_backend_set_discard_data(Object *o, bool value,
     MEMORY_BACKEND_FILE(o)->discard_data = value;
 }
 
+static void file_memory_backend_get_align(Object *o, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+    uint64_t val = fb->align;
+
+    visit_type_size(v, name, &val, errp);
+}
+
+static void file_memory_backend_set_align(Object *o, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+    Error *local_err = NULL;
+    uint64_t val;
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(&local_err, "cannot change property value");
+        goto out;
+    }
+
+    visit_type_size(v, name, &val, &local_err);
+    if (local_err) {
+        goto out;
+    }
+    fb->align = val;
+
+ out:
+    error_propagate(errp, local_err);
+}
+
 static void file_backend_unparent(Object *obj)
 {
     HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -145,6 +180,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
     object_class_property_add_str(oc, "mem-path",
         get_mem_path, set_mem_path,
         &error_abort);
+    object_class_property_add(oc, "align", "int",
+        file_memory_backend_get_align,
+        file_memory_backend_set_align,
+        NULL, NULL, &error_abort);
 }
 
 static void file_backend_instance_finalize(Object *o)