Embedded Controller chip could commonly be found
at platforms for laptop, it generally does
power management, keyboard and mouse simulation,
ACPI defined operation, low-speed devices handling ...
It talks with OS via io-port 0x60/0x54, 0x62/0x66,
the first pair is for i8042 compatible, the last
pair is for ACPI functions.
As QEMU has done power management, i8042 emulation,
this RFC patch will only focus on ACPI functions.
you can find standard description a ACPI SPEC
"ACPI Embedded Controller Interface Specification"