[Qemu-devel] [PATCH v4 2/3] Add memfd based hostmem

[Marc-André Lureau]

Add a new memory backend, similar to hostmem-file, except that it
doesn't need to create files. It also enforces memory sealing.

This backend is mainly useful for sharing the memory with other
processes.

Note that Linux supports transparent huge-pages of shmem/memfd memory
since 4.8. It is relatively easier to set up THP than a dedicate
hugepage mount point by using "madvise" in
/sys/kernel/mm/transparent_hugepage/shmem_enabled.

Usage:
-object memory-backend-memfd,id=mem1,size=1G

Signed-off-by: Marc-André Lureau <address@hidden>
---
 backends/hostmem-memfd.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++
 backends/Makefile.objs   |  2 ++
 qemu-options.hx          | 11 ++++++++
 3 files changed, 80 insertions(+)
 create mode 100644 backends/hostmem-memfd.c

diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
new file mode 100644
index 0000000000..13d300d9ad
--- /dev/null
+++ b/backends/hostmem-memfd.c
@@ -0,0 +1,67 @@
+/*
+ * QEMU host memfd memory backend
+ *
+ * Copyright (C) 2016 Red Hat Inc
+ *
+ * Authors:
+ *   Marc-André Lureau <address@hidden>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "sysemu/hostmem.h"
+#include "sysemu/sysemu.h"
+#include "qom/object_interfaces.h"
+#include "qemu/memfd.h"
+#include "qapi/error.h"
+
+#define TYPE_MEMORY_BACKEND_MEMFD "memory-backend-memfd"
+
+static void
+memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
+{
+    int fd;
+
+    if (!backend->size) {
+        error_setg(errp, "can't create backend with size 0");
+        return;
+    }
+
+    if (!memory_region_size(&backend->mr)) {
+        backend->force_prealloc = mem_prealloc;
+        fd = qemu_memfd_create(TYPE_MEMORY_BACKEND_MEMFD,
+                               backend->size,
+                               F_SEAL_GROW | F_SEAL_SHRINK | F_SEAL_SEAL,
+                               true);
+        if (fd == -1) {
+            error_setg(errp, "can't allocate memfd backend");
+            return;
+        }
+        memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
+            object_get_canonical_path(OBJECT(backend)),
+            backend->size, true, fd, errp);
+    }
+}
+
+static void
+memfd_backend_class_init(ObjectClass *oc, void *data)
+{
+    HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
+
+    bc->alloc = memfd_backend_memory_alloc;
+}
+
+static const TypeInfo memfd_backend_info = {
+    .name = TYPE_MEMORY_BACKEND_MEMFD,
+    .parent = TYPE_MEMORY_BACKEND,
+    .class_init = memfd_backend_class_init,
+};
+
+static void register_types(void)
+{
+    type_register_static(&memfd_backend_info);
+}
+
+type_init(register_types);
diff --git a/backends/Makefile.objs b/backends/Makefile.objs
index 0400799efd..67eeeba5fc 100644
--- a/backends/Makefile.objs
+++ b/backends/Makefile.objs
@@ -8,3 +8,5 @@ common-obj-$(CONFIG_LINUX) += hostmem-file.o
 
 common-obj-y += cryptodev.o
 common-obj-y += cryptodev-builtin.o
+
+common-obj-$(CONFIG_LINUX) += hostmem-memfd.o
diff --git a/qemu-options.hx b/qemu-options.hx
index 30c4f9850f..6db76cf54f 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3974,6 +3974,17 @@ The @option{share} boolean option determines whether the 
memory
 region is marked as private to QEMU, or shared. The latter allows
 a co-operating external process to access the QEMU memory region.
 
address@hidden -object memory-backend-memfd,address@hidden,address@hidden
+
+Creates an anonymous memory file backend object, which can be used to
+share the memory with a co-operating external process. The memory is
+allocated with memfd and sealing. (Linux only)
+
+The @option{id} parameter is a unique ID that will be used to
+reference this memory region when configuring the @option{-numa}
+argument. The @option{size} option provides the size of the memory
+region, and accepts common suffixes, eg @option{500M}.
+
 @item -object rng-random,address@hidden,address@hidden/dev/random}
 
 Creates a random number generator backend which obtains entropy from
-- 
2.13.1.395.gf7b71de06