memory: Introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()

Let's introduce RAM_NORESERVE, allowing mmap'ing with MAP_NORESERVE. The
new flag has the following semantics:

"
RAM is mmap-ed with MAP_NORESERVE. When set, reserving swap space (or huge
pages if applicable) is skipped: will bail out if not supported. When not
set, the OS will do the reservation, if supported for the memory type.
"

Allow passing it into:
- memory_region_init_ram_nomigrate()
- memory_region_init_resizeable_ram()
- memory_region_init_ram_from_file()

... and teach qemu_ram_mmap() and qemu_anon_ram_alloc() about the flag.
Bail out if the flag is not supported, which is the case right now for
both, POSIX and win32. We will add Linux support next and allow specifying
RAM_NORESERVE via memory backends.

The target use case is virtio-mem, which dynamically exposes memory
inside a large, sparse memory area to the VM.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-9-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
index 37a38c4..c2c7fe5 100644
--- a/include/qemu/osdep.h
+++ b/include/qemu/osdep.h
@@ -362,7 +362,8 @@ extern "C" {
 int qemu_daemon(int nochdir, int noclose);
 void *qemu_try_memalign(size_t alignment, size_t size);
 void *qemu_memalign(size_t alignment, size_t size);
-void *qemu_anon_ram_alloc(size_t size, uint64_t *align, bool shared);
+void *qemu_anon_ram_alloc(size_t size, uint64_t *align, bool shared,
+                          bool noreserve);
 void qemu_vfree(void *ptr);
 void qemu_anon_ram_free(void *ptr, size_t size);
 
@@ -383,6 +384,12 @@ void qemu_anon_ram_free(void *ptr, size_t size);
  */
 #define QEMU_MAP_SYNC       (1 << 2)
 
+/*
+ * Use MAP_NORESERVE to skip reservation of swap space (or huge pages if
+ * applicable). Bail out if not supported/effective.
+ */
+#define QEMU_MAP_NORESERVE  (1 << 3)
+
 
 #define QEMU_MADV_INVALID -1