hv-balloon: use get_min_alignment() to express 32 GiB alignment

Let's implement the get_min_alignment() callback for memory devices, and
copy for the device memory region the alignment of the host memory
region. This mimics what virtio-mem does, and allows for re-introducing
proper alignment checks for the memory region size (where we don't care
about additional device requirements) in memory device core.

Message-ID: <20240117135554.787344-2-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

hw/hyperv/hv-balloon.c

@@ -1477,22 +1477,7 @@ static void hv_balloon_ensure_mr(HvBalloon *balloon)
     balloon->mr = g_new0(MemoryRegion, 1);
     memory_region_init(balloon->mr, OBJECT(balloon), TYPE_HV_BALLOON,
                        memory_region_size(hostmem_mr));
-
-    /*
-     * The VM can indicate an alignment up to 32 GiB. Memory device core can
-     * usually only handle/guarantee 1 GiB alignment. The user will have to
-     * specify a larger maxmem eventually.
-     *
-     * The memory device core will warn the user in case maxmem might have to be
-     * increased and will fail plugging the device if there is not sufficient
-     * space after alignment.
-     *
-     * TODO: we could do the alignment ourselves in a slightly bigger region.
-     * But this feels better, although the warning might be annoying. Maybe
-     * we can optimize that in the future (e.g., with such a device on the
-     * cmdline place/size the device memory region differently.
-     */
-    balloon->mr->align = MAX(32 * GiB, memory_region_get_alignment(hostmem_mr));
+    balloon->mr->align = memory_region_get_alignment(hostmem_mr);
 }
 
 static void hv_balloon_free_mr(HvBalloon *balloon)
@@ -1654,6 +1639,25 @@ static MemoryRegion *hv_balloon_md_get_memory_region(MemoryDeviceState *md,
     return balloon->mr;
 }
 
+static uint64_t hv_balloon_md_get_min_alignment(const MemoryDeviceState *md)
+{
+    /*
+     * The VM can indicate an alignment up to 32 GiB. Memory device core can
+     * usually only handle/guarantee 1 GiB alignment. The user will have to
+     * specify a larger maxmem eventually.
+     *
+     * The memory device core will warn the user in case maxmem might have to be
+     * increased and will fail plugging the device if there is not sufficient
+     * space after alignment.
+     *
+     * TODO: we could do the alignment ourselves in a slightly bigger region.
+     * But this feels better, although the warning might be annoying. Maybe
+     * we can optimize that in the future (e.g., with such a device on the
+     * cmdline place/size the device memory region differently.
+     */
+    return 32 * GiB;
+}
+
 static void hv_balloon_md_fill_device_info(const MemoryDeviceState *md,
                                            MemoryDeviceInfo *info)
 {
@@ -1766,5 +1770,6 @@ static void hv_balloon_class_init(ObjectClass *klass, void *data)
     mdc->get_memory_region = hv_balloon_md_get_memory_region;
     mdc->decide_memslots = hv_balloon_decide_memslots;
     mdc->get_memslots = hv_balloon_get_memslots;
+    mdc->get_min_alignment = hv_balloon_md_get_min_alignment;
     mdc->fill_device_info = hv_balloon_md_fill_device_info;
 }