HPET on QEmu

[giszo]

I just wrote a little code for HPET and tested under QEmu. It detects the address of the HPET registers properly according to the appropriate ACPI table.

After that I tried to disable the HPET timer with the -no-hpet parameter of QEmu, but the problem is that the HPET ACPI table is still present and valid according to its checksum. My question is that how should I detect that the HPET is disabled?

[giszo]

After checking some of the HPET registers for invalid values, such as the ID register with 0 value, I was able to figure out that the HPET is "invalid".

BTW, it would be better to not even include the HPET ACPI table if it's disabled ...

[tantrikwizard]

I don't see anything in the wiki on hpet. (Perhaps my blackberry is wacking out). If you could include ur findings for others it would be appreciated.

[jal]

BTW, it would be better to not even include the HPET ACPI table if it's disabled ...

That could be a bug in QEmu. Although I can imagine that if the HPETs can be enabled by software (BIOS?) that they'd be included in the ACPI tables anyway.


JAL

[Love4Boobies]

The tables are also generated by the BIOS and I'm not sure what should happen on real hardware. I'm just guessing here but if there was a real machine with a HPET that the BIOS doesn't put in the HPET mode (i.e., runs in PIT compatibility mode), the OS could read the ACPI tables and know it's a HPET and set it in the right mode. However, I doubt QEMU will actually emulate a HPET in PIT mode judging by the flag (i.e., "-no-hpet"). Thus, it's probably a bug in QEMU.

Unfortunately this kind of behavior is probably not formally specified anywhere. As if buggy ACPI tables weren't enough...