Re: [PATCH 2/4] migration: Introduce VMStateOffset

[Peter Xu <peterx@redhat.com>]

On Fri, Jul 31, 2026 at 11:02:27AM -0300, Fabiano Rosas wrote:
> Hi Vladimir, I've been looking at this, could you clarify which vmstates
> do you think we could merge? I don't see it, either VARRAY vs. VBUFFER
> or VARRAY vs. BUFFER, also ARRAY vs. BUFFER doesn't seem to work.
> 
> One main point of difference is the size_offset/num_offset variants are
> only known at load-time, so we can't convert them between each other at
> build time because the either the total size or num will not be know.

Something like this?

NOTE: I hid a fix to a comment that is irrelevant.. which I mentioned @size
and @size_offset can't co-exist, but it can when VMS_MULTIPLY is set.  I
had a vague feeling we can further clean vmstate flags.

diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
index 24631fd678..3a02709650 100644
--- a/include/migration/vmstate.h
+++ b/include/migration/vmstate.h
@@ -69,17 +69,15 @@ enum VMStateFlags {
      * }). Dereference the pointer before using it as basis for
      * further pointer arithmetic (see e.g. VMS_ARRAY). Does not
      * affect the meaning of VMStateField.num_offset or
-     * VMStateField.size_offset; see VMS_VARRAY and VMS_VBUFFER for
-     * those.
+     * VMStateField.size_offset; see VMS_VARRAY for those.
      */
     VMS_POINTER          = 0x002,
 
     /* The field is an array of fixed size. VMStateField.num contains
      * the number of entries in the array. The size of each entry is
      * given by VMStateField.size and / or opaque +
-     * VMStateField.size_offset; see VMS_VBUFFER and
-     * VMS_MULTIPLY. Each array entry will be processed individually
-     * (VMStateField.info.get()/put() if VMS_STRUCT is not set,
+     * VMStateField.size_offset. Each array entry will be processed
+     * individually (VMStateField.info.get()/put() if VMS_STRUCT is not set,
      * recursion into VMStateField.vmsd if VMS_STRUCT is set). May not
      * be combined with VMS_VARRAY.
      */
@@ -108,18 +106,10 @@ enum VMStateFlags {
      */
     VMS_ARRAY_OF_POINTER = 0x040,
 
-    /* The size of the individual entries (a single array entry if
-     * VMS_ARRAY or VMS_VARRAY are set, or the field itself if
-     * neither is set) is variable (i.e. not known at compile-time),
-     * but the same for all entries. Use the int32_t at opaque +
-     * VMStateField.size_offset (subject to VMS_MULTIPLY) to determine
-     * the size of each (and every) entry. */
-    VMS_VBUFFER          = 0x100,
-
     /* Multiply the entry size given by the int32_t at opaque +
-     * VMStateField.size_offset (see VMS_VBUFFER description) with
-     * VMStateField.size to determine the number of bytes to be
-     * allocated. Only valid in combination with VMS_VBUFFER. */
+     * VMStateField.size_offset with VMStateField.size to determine the
+     * number of bytes to be allocated.
+     */
     VMS_MULTIPLY         = 0x200,
 
     /* Fail loading the serialised VM state if this field is missing
@@ -181,8 +171,8 @@ struct VMStateField {
 
     /*
      * @size or @size_offset specifies the size of the element embeded in
-     * the field.  Only one of them should be present never both.  When
-     * @size_offset is used together with VMS_VBUFFER, it means the size is
+     * the field.  Only one of them should be present never both, except
+     * VMS_MULTIPLY.  When @size_offset is used, it means the size is
      * dynamic calculated instead of a constant.
      *
      * When the field is an array of any type, this stores the size of one
@@ -696,7 +686,7 @@ extern const VMStateInfo vmstate_info_g_byte_array;
     .size_offset  = vmstate_field_offset(_state, _field_size),       \
     .size         = (_multiply),                                      \
     .info         = &vmstate_info_buffer,                            \
-    .flags        = VMS_VBUFFER|VMS_POINTER|VMS_MULTIPLY,            \
+    .flags        = VMS_VARRAY|VMS_POINTER|VMS_MULTIPLY,              \
     .offset       = offsetof(_state, _field),                        \
 }
 
@@ -706,7 +696,7 @@ extern const VMStateInfo vmstate_info_g_byte_array;
     .field_exists = (_test),                                         \
     .size_offset  = vmstate_field_offset(_state, _field_size),       \
     .info         = &vmstate_info_buffer,                            \
-    .flags        = VMS_VBUFFER|VMS_POINTER,                         \
+    .flags        = VMS_VARRAY|VMS_POINTER,                          \
     .offset       = offsetof(_state, _field),                        \
 }
 
@@ -720,7 +710,7 @@ extern const VMStateInfo vmstate_info_g_byte_array;
     .field_exists = (_test),                                         \
     .size_offset  = vmstate_field_offset(_state, _field_size),       \
     .info         = &vmstate_info_buffer,                            \
-    .flags        = VMS_VBUFFER|VMS_POINTER|VMS_ALLOC,               \
+    .flags        = VMS_VARRAY|VMS_POINTER|VMS_ALLOC,                \
     .offset       = offsetof(_state, _field),                        \
 }
 
@@ -798,7 +788,7 @@ extern const VMStateInfo vmstate_info_g_byte_array;
     .version_id   = (_version),                                      \
     .size_offset  = vmstate_field_offset(_state, _field_size),       \
     .info         = &vmstate_info_bitmap,                            \
-    .flags        = VMS_VBUFFER|VMS_POINTER,                         \
+    .flags        = VMS_VARRAY|VMS_POINTER,                          \
     .offset       = offsetof(_state, _field),                        \
 }
 
@@ -1200,9 +1190,6 @@ extern const VMStateInfo vmstate_info_g_byte_array;
 #define VMSTATE_BUFFER_START_MIDDLE(_f, _s, _start) \
     VMSTATE_BUFFER_START_MIDDLE_V(_f, _s, _start, 0)
 
-#define VMSTATE_PARTIAL_VBUFFER(_f, _s, _size)                        \
-    VMSTATE_VBUFFER(_f, _s, 0, NULL, _size)
-
 #define VMSTATE_PARTIAL_VBUFFER_UINT32(_f, _s, _size)                        \
     VMSTATE_VBUFFER_UINT32(_f, _s, 0, NULL, _size)
 
diff --git a/migration/vmstate.c b/migration/vmstate.c
index bc8eb3d3ca..f7363a8776 100644
--- a/migration/vmstate.c
+++ b/migration/vmstate.c
@@ -102,7 +102,7 @@ static uint64_t vmstate_n_elems(void *opaque, const VMStateField *field)
 
     if (field->flags & VMS_ARRAY) {
         n_elems = field->num;
-    } else if (field->flags & VMS_VARRAY) {
+    } else if (field->num_offset.size) {
         n_elems = vmstate_read_from_offset(opaque, &field->num_offset);
     }
 
@@ -114,17 +114,17 @@ static uint64_t vmstate_size(void *opaque, const VMStateField *field)
 {
     uint64_t size;
 
-    if (field->flags & VMS_VBUFFER) {
-        size = vmstate_read_from_offset(opaque, &field->size_offset);
-        if (field->flags & VMS_MULTIPLY) {
-            size *= field->size;
-        }
-    } else if (field->flags & VMS_ARRAY_OF_POINTER) {
+    if (field->flags & VMS_ARRAY_OF_POINTER) {
         /*
          * For an array of pointer, the each element is always size of a
          * host pointer.
          */
         size = sizeof(void *);
+    } else if (field->size_offset.size) {
+        size = vmstate_read_from_offset(opaque, &field->size_offset);
+        if (field->flags & VMS_MULTIPLY) {
+            size *= field->size;
+        }
     } else {
         size = field->size;
     }
-- 
2.54.0

-- 
Peter Xu