diff --git a/hw/misc/pci-testdev.c b/hw/misc/pci-testdev.c
index 5df9089218..2f2e989778 100644
--- a/hw/misc/pci-testdev.c
+++ b/hw/misc/pci-testdev.c
@@ -239,6 +239,7 @@ static void pci_testdev_realize(PCIDevice *pci_dev, Error **errp)
     uint8_t *pci_conf;
     char *name;
     int r, i;
+    bool fastmmio = kvm_ioeventfd_any_length_enabled();
 
     pci_conf = pci_dev->config;
 
@@ -261,8 +262,12 @@ static void pci_testdev_realize(PCIDevice *pci_dev, Error **errp)
         memcpy(test->hdr->name, name, strlen(name) + 1);
         g_free(name);
         test->hdr->offset = cpu_to_le32(IOTEST_SIZE(i) + i * IOTEST_ACCESS_WIDTH);
-        test->size = IOTEST_ACCESS_WIDTH;
         test->match_data = strcmp(IOTEST_TEST(i), "wildcard-eventfd");
+        if (fastmmio && IOTEST_IS_MEM(i) && !test->match_data) {
+            test->size = 0;
+        } else {
+            test->size = IOTEST_ACCESS_WIDTH;
+        }
         test->hdr->test = i;
         test->hdr->data = test->match_data ? IOTEST_DATAMATCH : IOTEST_NOMATCH;
         test->hdr->width = IOTEST_ACCESS_WIDTH;