hostmem-file: add offset option

Add an option for hostmem-file to start the memory object at an offset
into the target file. This is useful if multiple memory objects reside
inside the same target file, such as a device node.

In particular, it's useful to map guest memory directly into /dev/mem
for experimentation.

To make this work consistently, also fix up all places in QEMU that
expect fd offsets to be 0.

Signed-off-by: Alexander Graf <graf@amazon.com>
Message-Id: <20230403221421.60877-1-graf@amazon.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 39 insertions, 1 deletions

diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
index 25141283c4..38ea65bec5 100644
--- a/backends/hostmem-file.c
+++ b/backends/hostmem-file.c
@@ -27,6 +27,7 @@ struct HostMemoryBackendFile {
 
     char *mem_path;
     uint64_t align;
+    uint64_t offset;
     bool discard_data;
     bool is_pmem;
     bool readonly;
@@ -58,7 +59,8 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
     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);
+                                     fb->mem_path, fb->offset, fb->readonly,
+                                     errp);
     g_free(name);
 #endif
 }
@@ -125,6 +127,36 @@ static void file_memory_backend_set_align(Object *o, Visitor *v,
     fb->align = val;
 }
 
+static void file_memory_backend_get_offset(Object *o, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+    uint64_t val = fb->offset;
+
+    visit_type_size(v, name, &val, errp);
+}
+
+static void file_memory_backend_set_offset(Object *o, Visitor *v,
+                                          const char *name, void *opaque,
+                                          Error **errp)
+{
+    HostMemoryBackend *backend = MEMORY_BACKEND(o);
+    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(o);
+    uint64_t val;
+
+    if (host_memory_backend_mr_inited(backend)) {
+        error_setg(errp, "cannot change property '%s' of %s", name,
+                   object_get_typename(o));
+        return;
+    }
+
+    if (!visit_type_size(v, name, &val, errp)) {
+        return;
+    }
+    fb->offset = val;
+}
+
 #ifdef CONFIG_LIBPMEM
 static bool file_memory_backend_get_pmem(Object *o, Error **errp)
 {
@@ -197,6 +229,12 @@ file_backend_class_init(ObjectClass *oc, void *data)
         file_memory_backend_get_align,
         file_memory_backend_set_align,
         NULL, NULL);
+    object_class_property_add(oc, "offset", "int",
+        file_memory_backend_get_offset,
+        file_memory_backend_set_offset,
+        NULL, NULL);
+    object_class_property_set_description(oc, "offset",
+        "Offset into the target file (ex: 1G)");
 #ifdef CONFIG_LIBPMEM
     object_class_property_add_bool(oc, "pmem",
         file_memory_backend_get_pmem, file_memory_backend_set_pmem);