Payload disk is a composite disk image referencing host APEXes and an APK so that microdroid mounts/activates APK/APEXes and executes a binary within the APK.
Payload disk is created by VirtualizationService Service when starting a VM.
Payload disk has 1 + N(number of APEX/APK payloads) partitions.
The first partition is a "payload-metadata" partition which describes other partitions. And APEXes and an APK are following as separate partitions.
For now, the order of partitions are important.
- partition 1: Metadata partition
- partition 2 ~ n: APEX payloads
- partition n+1, n+2: APK payload and its idsig
It's subject to change in the future, though.
Metadata partition provides description of the other partitions and the location for VM payload configuration.
The partition is a protobuf message prefixed with the size of the message.
| offset | size | description |
|---|---|---|
| 0 | 4 | Header. unsigned int32: body length(L) in big endian |
| 4 | L | Body. A protobuf message. schema |
Each payload partition presents APEX or APK passed from the host.
The size of a payload partition must be a multiple of 4096 bytes.