[PATCH v1 9/9] hostmem: wire up RAM_NORESERVE via "reserve" property

[David Hildenbrand <david@redhat.com>]

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 POSIX supports setting the flag (and Linux support is checked at
runtime, depending on the setting of "/proc/sys/vm/overcommit_memory").
Windows will bail out.

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.

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       | 33 +++++++++++++++++++++++++++++++++
 include/sysemu/hostmem.h |  2 +-
 5 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 733408e076..a2d7e83f9c 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 54db08de0d..5412e67bad 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, 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 be0c3b079f..c979595115 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 +522,11 @@ 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 under Linux) for the whole memory"
+        " backend, if supported by the OS.");
     object_class_property_add_bool(oc, "x-use-canonical-path-for-ramblock-id",
         host_memory_backend_get_use_canonical_path,
         host_memory_backend_set_use_canonical_path);
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;
-- 
2.29.2