Why do we need start_devices here, rather than using _vm_as?

I think the difference here between the previous _ qualifying variable to kill unused is those two fields are not read anywhere, but here vm_as will be used if host-device is enabled. Actually this kind of writing is not elegant at least, we can work on refactoring it later

Yep, sure.

What is this feature used for?

I think the feature attribute is useless for this test, since impl Serialize for MacAddr doesn't have the attribute, right?

    #[cfg(feature = "with-serde")]
    #[test]
    fn test_mac_addr_serialization_and_deserialization() {
        let mac: MacAddr =
            serde_json::from_str("\"12:34:56:78:9a:bc\"").expect("MacAddr deserialization failed.");

        let bytes = mac.get_bytes();
        assert_eq!(bytes, [0x12u8, 0x34, 0x56, 0x78, 0x9a, 0xbc]);

        let s = serde_json::to_string(&mac).expect("MacAddr serialization failed.");
        assert_eq!(s, "\"12:34:56:78:9a:bc\"");
    }

impl Serialize for MacAddr {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        self.to_string().serialize(serializer)
    }
}

We either remove the feature assertion on top of test_mac_addr_serialization_and_deserialization, or we add this feature to features section, otherwise test_mac_addr_serialization_and_deserialization will not be triggering 🤔

I think we should simply remove #[cfg(feature = "with-serde")] from test_mac_addr_serialization_and_deserialization.

Why do you add the vm_memory here?

It documented in the commit message, MmapRegion is only need when virtio-fs feature is enabled during testing

Perhaps you can use this instead:

use vm_memory::{GuestAddress, GuestUsize};
#[cfg(feature = "virtio-fs")]
use vm_memory::MmapRegion;

Sure, I thought the same way before, and I just saw only one place which referenced MmapRegion which makes me tend to think it would be a bit overkill. Either way is fine actually, I will amend it