diff --git a/hw/misc/unimp.c b/hw/misc/unimp.c
index b4b318db1c..6cfc5727f0 100644
--- a/hw/misc/unimp.c
+++ b/hw/misc/unimp.c
@@ -23,8 +23,8 @@ static uint64_t unimp_read(void *opaque, hwaddr offset, unsigned size)
     UnimplementedDeviceState *s = UNIMPLEMENTED_DEVICE(opaque);
 
     qemu_log_mask(LOG_UNIMP, "%s: unimplemented device read  "
-                  "(size %d, offset 0x%" HWADDR_PRIx ")\n",
-                  s->name, size, offset);
+                  "(size %d, offset 0x%0*" HWADDR_PRIx ")\n",
+                  s->name, size, s->offset_fmt_width, offset);
     return 0;
 }
 
@@ -34,9 +34,9 @@ static void unimp_write(void *opaque, hwaddr offset,
     UnimplementedDeviceState *s = UNIMPLEMENTED_DEVICE(opaque);
 
     qemu_log_mask(LOG_UNIMP, "%s: unimplemented device write "
-                  "(size %d, offset 0x%" HWADDR_PRIx
+                  "(size %d, offset 0x%0*" HWADDR_PRIx
                   ", value 0x%0*" PRIx64 ")\n",
-                  s->name, size, offset, size << 1, value);
+                  s->name, size, s->offset_fmt_width, offset, size << 1, value);
 }
 
 static const MemoryRegionOps unimp_ops = {
@@ -63,6 +63,8 @@ static void unimp_realize(DeviceState *dev, Error **errp)
         return;
     }
 
+    s->offset_fmt_width = DIV_ROUND_UP(64 - clz64(s->size - 1), 4);
+
     memory_region_init_io(&s->iomem, OBJECT(s), &unimp_ops, s,
                           s->name, s->size);
     sysbus_init_mmio(SYS_BUS_DEVICE(s), &s->iomem);
diff --git a/include/hw/misc/unimp.h b/include/hw/misc/unimp.h
index 4c1d13c9bf..c63968a2cd 100644
--- a/include/hw/misc/unimp.h
+++ b/include/hw/misc/unimp.h
@@ -20,6 +20,7 @@
 typedef struct {
     SysBusDevice parent_obj;
     MemoryRegion iomem;
+    unsigned offset_fmt_width;
     char *name;
     uint64_t size;
 } UnimplementedDeviceState;