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Subjecti8k_smm_func() takes enormous of time to execute
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Hi, Jonathan, Mario et al.

I've noticed that querying Dell Vostro 3360 hwmon sensor freezes the system
completely (like, really completely, even mouse cursor does not move, and
sound playback stops) for approx. half of a second. Then, system recovers and
runs as usual. Also, sensor readings are okay, for instance:

===
$ sensors dell_smm-virtual-0
dell_smm-virtual-0
Adapter: Virtual device
Processor Fan: 5615 RPM
CPU: +51.0°C
Ambient: +36.0°C
Other: +63.0°C
SODIMM: +41.0°C
===

So, I've used trace-cmd to check what takes that much amount of time to
execute, and got this:

===
# trace-cmd record -p function_graph -l i8k_smm -F sensors dell_smm-virtual-0
$ trace-cmd report
...
sensors-23694 [002] 89099.214369: funcgraph_entry: # 503440.746 us |
i8k_smm();
...
===

Clearly, 0.5 s delay.

Looking at i8k_smm(), it calls i8k_smm_func() on 0th CPU:

===
232 ret = smp_call_on_cpu(0, i8k_smm_func, regs, true);
===

which, in turn, does some asm magic.

I know that SMM is kinda "black box", and kernel has little to do with it, but
I think that under Windows, for instance, it would work without freezes. So,
likely, querying SMM might be done differently.

I do not know how to approach this issue, thus asking for help/advice. Also,
CCing Jonathan since the comment before asm magic says this:

===
137 * Call the System Management Mode BIOS. Code provided by Jonathan
Buzzard.
===

This was also reported in various places before, for instance, [1], but
unfortunately, without any solution.

Thanks.

Regards,
Oleksandr

[1] https://bugs.launchpad.net/ubuntu/+source/acpi/+bug/10490

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