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CSME should be enabled when S0ix is used for power savings during suspend. TGL-U is just one case where we default as our TGL-U models do not support S3. Other models may use S0ix as well, such as darp8 and darp9 due to new batches just not working with S3 anymore [1]. [1]: https://github.com/system76/firmware-open/issues/507 Signed-off-by: Tim Crawford <tcrawford@system76.com>
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Intel Management Engine
Intel Management Engine is a proprietary, mostly undocumented, firmware system that provides many extraneous features that are generally not usable or useful to our users, with multiple known vulnerabilities that compromise the entire system.
The Intel ME is required (since Nehalem, 2008), so cannot be removed. The me_cleaner project is able to remove non-essential components, but does not support the ME version used on many of our systems. Instead, we send a HECI command to tell the Intel ME to disable runtime components during early boot.
Most Intel-based machines from System76 come with the IME disabled.
Configuring
The IME can be enabled or disabled via the coreboot CMOS option me_state.
The value can be set using coreboot's nvramtool.
make -C coreboot/util/nvramtool
sudo ./coreboot/util/nvramtool/nvramtool -w me_state={Enable,Disable}
A restart is required for the change to take effect. On the boot after changing the value, the system will perform a global reset (power off again) to complete the change and ensure the IME is operating in a valid state.
Checking the state
coreboot will log some IME data to cbmem during startup. This can be used to check if it is in the correct state.
make -C coreboot/util/cbmem
sudo ./coreboot/util/cbmem/cbmem -c
When disabled it will report:
ME: Current Working State : 4
ME: Current Operation State : 1
ME: Current Operation Mode : 3
ME: Error Code : 2
When enabled it will report:
ME: Current Working State : 5
ME: Current Operation State : 1
ME: Current Operation Mode : 0
ME: Error Code : 0
S0ix
S0ix (Modern Standby, s2idle) requires all CPU, PCH, and PCIe devices to have ACPI defined low power states. The CPU has numerous states for low power, with the lowest being C10. In order to reach this C10 state, the CSME must report that it is in a low power state.
Disabling the CSME with the HAP bit or HECI command keeps the CPU in the C8 state. This nearly triples the power usage in S0ix suspend, from around 1 watt to around 3 watts.
TGL-U removed support for S3 and requires S0ix.