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rdtscp instruction


Both AMD and Intel CPUs support the RDTSCP instruction.

'cpuctl indentify 0'

prints it in the features3 line.

The differences between RDTSC and RDTSCP are:

- RDTSCP is a serializing instruction
- RDTSCP additionally loads the lower 32bits
  of MSR_TSC_AUX into ecx register.

The MSR_TSC_AUX (0xc0000103) is read-write
for the OS and can be filled with anything
meaningful to the OS (i.e. processor ID).
The upper 32bits are reserved.
The MSR exists for each CPU-core.

The question is: What useful information
can NetBSD store into MSR_TSC_AUX ?


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