Because CPUID returns the APIC ID, and I was under the impression that the cpu numbers
smp_processor_id() were dynamically allocated and didn't necessarily match the
APIC ID.
Correct, but one can use a mapping table.
Yes, it's only needed on HLT and cpufreq change.
The code here is to force a "resynch" with the HPET if we've done a HLT. It has to be done before we switch to any userland thread that might use the per-cpu vxtime. switch_to() seemed like the most natural place to put this.
I don't think so. The natural place after HLT is in the idle loop or better in idle notifiers[1] and after cpufreq is in the appropiate cpufreq notifiers.
[1] unfortunately they are still subtly broken in .19, but will be fixed
in .20
>A cow-orker suggested that we use SIDT and encode the CPU number in the limit of the IDT, which should be even faster than LSL.
Possible yes. Did you time it?
But then we would make the IDT variable length in memory? While
the CPUs probably won't care some Hypervisors seem to be picky
about these limits. LSL still seems somewhat safer.
I couldn't figure out how to tell if a context switch has happened from userland. I tried putting a per-cpu context switch count, but I couldn't figure out how to get it atomically along with the CPU number..
It's tricky. That is why we asked for RDTSCP.