Is there an `x86` instruction to tell which core the instruction is being run on?

Question

When I cat /proc/cpuinfo, I see 8 cores, with ID\'s from 0 to 7.

Is there an x86 instruction that will report th

Answer 1

taskset + __rdtscp runnable example

And at last, for those that want to have some fun with x86 intrinsics + taskset:

rdtscp.c

#include 
#include 
#include 
#include 

#include 

int main(void) {
    uint32_t pid;
    printf("0x%016" PRIX64 "\n", (uint64_t)__rdtscp(&pid));
    printf("0x%08" PRIX32 "\n", pid);
    return EXIT_SUCCESS;
}

GitHub upstream.

then compile and run while controlling which core it runs on with taskset:

gcc -ggdb3 -O0 -std=c99 -Wall -Wextra -pedantic -o rdtscp.out rdtscp.c
./taskset -c 0 ./rdtscp.out
./taskset -c 1 ./rdtscp.out

then for each run, the second line, which shows the CPU ID, matches the value set by taskset.

Tested in Ubuntu 19.04 amd64 with an Intel Core i7-7820HQ.