[v4,20/29] hostmem: allow preallocation of any memory region

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index b8df933..d3a7ef3 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -9,7 +9,9 @@ 
  * 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-common.h"
 #include "sysemu/hostmem.h"
+#include "sysemu/sysemu.h"
 #include "qom/object_interfaces.h"
 
 /* hostmem-file.c */
@@ -46,6 +48,7 @@  file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
     error_setg(errp, "-mem-path not supported on this host");
 #else
     if (!memory_region_size(&backend->mr)) {
+        backend->force_prealloc = mem_prealloc;
         memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
                                  object_get_canonical_path(OBJECT(backend)),
                                  backend->size,
diff --git a/backends/hostmem.c b/backends/hostmem.c
index fa306b4..e437275 100644
--- a/backends/hostmem.c
+++ b/backends/hostmem.c
@@ -105,6 +105,41 @@  static void host_memory_backend_set_dump(Object *obj, bool value, Error **errp)
     }
 }
 
+static bool host_memory_backend_get_prealloc(Object *obj, Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+
+    return backend->prealloc || backend->force_prealloc;
+}
+
+static void host_memory_backend_set_prealloc(Object *obj, bool value,
+                                             Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+
+    if (backend->force_prealloc) {
+        if (value) {
+            error_setg(errp,
+                       "remove -mem-prealloc to use the prealloc property");
+            return;
+        }
+    }
+
+    if (!memory_region_size(&backend->mr)) {
+        backend->prealloc = value;
+        return;
+    }
+
+    if (value && !backend->prealloc) {
+        int fd = memory_region_get_fd(&backend->mr);
+        void *ptr = memory_region_get_ram_ptr(&backend->mr);
+        uint64_t sz = memory_region_size(&backend->mr);
+
+        os_mem_prealloc(fd, ptr, sz);
+        backend->prealloc = true;
+    }
+}
+
 static void host_memory_backend_init(Object *obj)
 {
     HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -113,6 +148,7 @@  static void host_memory_backend_init(Object *obj)
                                        "mem-merge", true);
     backend->dump = qemu_opt_get_bool(qemu_get_machine_opts(),
                                       "dump-guest-core", true);
+    backend->prealloc = mem_prealloc;
 
     object_property_add_bool(obj, "merge",
                         host_memory_backend_get_merge,
@@ -120,6 +156,9 @@  static void host_memory_backend_init(Object *obj)
     object_property_add_bool(obj, "dump",
                         host_memory_backend_get_dump,
                         host_memory_backend_set_dump, NULL);
+    object_property_add_bool(obj, "prealloc",
+                        host_memory_backend_get_prealloc,
+                        host_memory_backend_set_prealloc, NULL);
     object_property_add(obj, "size", "int",
                         host_memory_backend_get_size,
                         host_memory_backend_set_size, NULL, NULL, NULL);
@@ -165,6 +204,9 @@  host_memory_backend_memory_complete(UserCreatable *uc, Error **errp)
         if (!backend->dump) {
             qemu_madvise(ptr, sz, QEMU_MADV_DONTDUMP);
         }
+        if (backend->prealloc) {
+            os_mem_prealloc(memory_region_get_fd(&backend->mr), ptr, sz);
+        }
     }
 }
 
diff --git a/exec.c b/exec.c
index 739f0cf..520d673 100644
--- a/exec.c
+++ b/exec.c
@@ -1432,6 +1432,13 @@  void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
 }
 #endif /* !_WIN32 */
 
+int qemu_get_ram_fd(ram_addr_t addr)
+{
+    RAMBlock *block = qemu_get_ram_block(addr);
+
+    return block->fd;
+}
+
 /* Return a host pointer to ram allocated with qemu_ram_alloc.
    With the exception of the softmmu code in this file, this should
    only be used for local memory (e.g. video ram) that the device owns,
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 82d7781..36226f7 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -534,6 +534,16 @@  bool memory_region_is_logging(MemoryRegion *mr);
 bool memory_region_is_rom(MemoryRegion *mr);
 
 /**
+ * memory_region_get_fd: Get a file descriptor backing a RAM memory region.
+ *
+ * Returns a file descriptor backing a file-based RAM memory region,
+ * or -1 if the region is not a file-based RAM memory region.
+ *
+ * @mr: the RAM or alias memory region being queried.
+ */
+int memory_region_get_fd(MemoryRegion *mr);
+
+/**
  * memory_region_get_ram_ptr: Get a pointer into a RAM memory region.
  *
  * Returns a host pointer to a RAM memory region (created with
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index f9518a6..d352f60 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -27,6 +27,7 @@  ram_addr_t qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
 ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
                                    MemoryRegion *mr);
 ram_addr_t qemu_ram_alloc(ram_addr_t size, MemoryRegion *mr);
+int qemu_get_ram_fd(ram_addr_t addr);
 void *qemu_get_ram_ptr(ram_addr_t addr);
 void qemu_ram_free(ram_addr_t addr);
 void qemu_ram_free_from_ptr(ram_addr_t addr);
diff --git a/include/sysemu/hostmem.h b/include/sysemu/hostmem.h
index ede5ec9..4cae673 100644
--- a/include/sysemu/hostmem.h
+++ b/include/sysemu/hostmem.h
@@ -53,6 +53,7 @@  struct HostMemoryBackend {
     /* protected */
     uint64_t size;
     bool merge, dump;
+    bool prealloc, force_prealloc;
 
     MemoryRegion mr;
 };
diff --git a/memory.c b/memory.c
index 310729a..bcef72b 100644
--- a/memory.c
+++ b/memory.c
@@ -1258,6 +1258,17 @@  void memory_region_reset_dirty(MemoryRegion *mr, hwaddr addr,
     cpu_physical_memory_reset_dirty(mr->ram_addr + addr, size, client);
 }
 
+int memory_region_get_fd(MemoryRegion *mr)
+{
+    if (mr->alias) {
+        return memory_region_get_fd(mr->alias);
+    }
+
+    assert(mr->terminates);
+
+    return qemu_get_ram_fd(mr->ram_addr & TARGET_PAGE_MASK);
+}
+
 void *memory_region_get_ram_ptr(MemoryRegion *mr)
 {
     if (mr->alias) {