[PATCH v7 10/15] hostmem: Wire up RAM_NORESERVE via "reserve" property

[David Hildenbrand]

Let's provide a way to control the use of RAM_NORESERVE via memory
backends using the "reserve" property which defaults to true (old
behavior).

Only Linux currently supports clearing the flag (and support is checked at
runtime, depending on the setting of "/proc/sys/vm/overcommit_memory").
Windows and other POSIX systems will bail out with "reserve=false".

The target use case is virtio-mem, which dynamically exposes memory
inside a large, sparse memory area to the VM. This essentially allows
avoiding to set "/proc/sys/vm/overcommit_memory == 0") when using
virtio-mem and also supporting hugetlbfs in the future.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine 
core
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 backends/hostmem-file.c  | 11 ++++++-----
 backends/hostmem-memfd.c |  1 +
 backends/hostmem-ram.c   |  1 +
 backends/hostmem.c       | 32 ++++++++++++++++++++++++++++++++
 include/sysemu/hostmem.h |  2 +-
 qapi/qom.json            | 10 ++++++++++
 6 files changed, 51 insertions(+), 6 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index b683da9daf..9d550e53d4 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -40,6 +40,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error 
**errp)
                object_get_typename(OBJECT(backend)));
 #else
     HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(backend);
+    uint32_t ram_flags;
     gchar *name;
 
     if (!backend->size) {
@@ -52,11 +53,11 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error 
**errp)
     }
 
     name = host_memory_backend_get_name(backend);
-    memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
-                                     name,
-                                     backend->size, fb->align,
-                                     (backend->share ? RAM_SHARED : 0) |
-                                     (fb->is_pmem ? RAM_PMEM : 0),
+    ram_flags = backend->share ? RAM_SHARED : 0;
+    ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+    ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
+    memory_region_init_ram_from_file(&backend->mr, OBJECT(backend), name,
+                                     backend->size, fb->align, ram_flags,
                                      fb->mem_path, fb->readonly, errp);
     g_free(name);
 #endif
diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
index 93b5d1a4cf..f3436b623d 100644
--- a/backends/hostmem-memfd.c
+++ b/backends/hostmem-memfd.c
@@ -55,6 +55,7 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error 
**errp)
 
     name = host_memory_backend_get_name(backend);
     ram_flags = backend->share ? RAM_SHARED : 0;
+    ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
     memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend), name,
                                    backend->size, ram_flags, fd, 0, errp);
     g_free(name);
diff --git a/backends/hostmem-ram.c b/backends/hostmem-ram.c
index 741e701062..b8e55cdbd0 100644
--- a/backends/hostmem-ram.c
+++ b/backends/hostmem-ram.c
@@ -29,6 +29,7 @@ ram_backend_memory_alloc(HostMemoryBackend *backend, Error 
**errp)
 
     name = host_memory_backend_get_name(backend);
     ram_flags = backend->share ? RAM_SHARED : 0;
+    ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
     memory_region_init_ram_flags_nomigrate(&backend->mr, OBJECT(backend), name,
                                            backend->size, ram_flags, errp);
     g_free(name);
diff --git a/backends/hostmem.c b/backends/hostmem.c
index c6c1ff5b99..58fdc1b658 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -217,6 +217,11 @@ static void host_memory_backend_set_prealloc(Object *obj, 
bool value,
     Error *local_err = NULL;
     HostMemoryBackend *backend = MEMORY_BACKEND(obj);
 
+    if (!backend->reserve && value) {
+        error_setg(errp, "'prealloc=on' and 'reserve=off' are incompatible");
+        return;
+    }
+
     if (!host_memory_backend_mr_inited(backend)) {
         backend->prealloc = value;
         return;
@@ -268,6 +273,7 @@ static void host_memory_backend_init(Object *obj)
     /* TODO: convert access to globals to compat properties */
     backend->merge = machine_mem_merge(machine);
     backend->dump = machine_dump_guest_core(machine);
+    backend->reserve = true;
     backend->prealloc_threads = 1;
 }
 
@@ -426,6 +432,28 @@ static void host_memory_backend_set_share(Object *o, bool 
value, Error **errp)
     backend->share = value;
 }
 
+static bool host_memory_backend_get_reserve(Object *o, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+
+    return backend->reserve;
+}
+
+static void host_memory_backend_set_reserve(Object *o, bool value, Error 
**errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property value");
+        return;
+    }
+    if (backend->prealloc && !value) {
+        error_setg(errp, "'prealloc=on' and 'reserve=off' are incompatible");
+        return;
+    }
+    backend->reserve = value;
+}
+
 static bool
 host_memory_backend_get_use_canonical_path(Object *obj, Error **errp)
 {
@@ -494,6 +522,10 @@ host_memory_backend_class_init(ObjectClass *oc, void *data)
         host_memory_backend_get_share, host_memory_backend_set_share);
     object_class_property_set_description(oc, "share",
         "Mark the memory as private to QEMU or shared");
+    object_class_property_add_bool(oc, "reserve",
+        host_memory_backend_get_reserve, host_memory_backend_set_reserve);
+    object_class_property_set_description(oc, "reserve",
+        "Reserve swap space (or huge pages) if applicable");
     /*
      * Do not delete/rename option. This option must be considered stable
      * (as if it didn't have the 'x-' prefix including deprecation period) as
diff --git a/include/sysemu/hostmem.h b/include/sysemu/hostmem.h
index df5644723a..9ff5c16963 100644
--- a/include/sysemu/hostmem.h
+++ b/include/sysemu/hostmem.h
@@ -64,7 +64,7 @@ struct HostMemoryBackend {
     /* protected */
     uint64_t size;
     bool merge, dump, use_canonical_path;
-    bool prealloc, is_mapped, share;
+    bool prealloc, is_mapped, share, reserve;
     uint32_t prealloc_threads;
     DECLARE_BITMAP(host_nodes, MAX_NODES + 1);
     HostMemPolicy policy;
diff --git a/qapi/qom.json b/qapi/qom.json
index cd0e76d564..4fa3137aab 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -545,6 +545,9 @@
 # @share: if false, the memory is private to QEMU; if true, it is shared
 #         (default: false)
 #
+# @reserve: if true, reserve swap space (or huge pages) if applicable
+#           default: true)
+#
 # @size: size of the memory region in bytes
 #
 # @x-use-canonical-path-for-ramblock-id: if true, the canoncial path is used
@@ -556,6 +559,12 @@
 #                                        false generally, but true for machine
 #                                        types <= 4.0)
 #
+# Note: prealloc=true and reserve=false cannot be set at the same time. With
+#       reserve=true, the behavior depends on the operating system: for 
example,
+#       Linux will not reserve swap space for shared file mappings --
+#       "not applicable". In contrast, reserve=false will bail out if it cannot
+#       be configured accordingly.
+#
 # Since: 2.1
 ##
 { 'struct': 'MemoryBackendProperties',
@@ -566,6 +575,7 @@
             '*prealloc': 'bool',
             '*prealloc-threads': 'uint32',
             '*share': 'bool',
+            '*reserve': 'bool',
             'size': 'size',
             '*x-use-canonical-path-for-ramblock-id': 'bool' } }
 
-- 
2.30.2