According to OSDev, to locate the Port I/O address of ACPI timer, we first open FADT table and check entries PM Timer Block Length and PM Timer Block Address. In my computer, PM Timer Block Address gives address 0x408 and it works correctly.
However, in the implementation of OVMF, the I/O address of ACPI timer is calculated as PMBA + 0x8. I search the internet and found no information about this way of calculation.
I'm wondering are both methods to decide ACPI timer address correct? If both are correct, where can I find definitions of information about the second way of calculation?
Firmware (e.g. OVMF) uses chipset specific methods to determine the IO port of the ACPI timer; and then constructs the FADT and fills it in so that an OS doesn't need to be chipset specific.
If you don't want to use FADT, then you can write a chipset specific driver for each chipset. For some cases (open source emulators) this may be relatively easy, and for some cases (proprietary and undocumented real hardware) this will be almost impossible.