Linux Kernel Module Cheat

Sources:

Try running the programs multiple times, and watch the value increase, and then try to correlate it with /proc/cpuinfo frequency!

while true; do sleep 1 && ./userland/arch/x86_64/rdtsc.out; done

RDTSC stores its output to EDX:EAX, even in 64-bit mode, top bits are zeroed out.

TODO: review this section, make a more controlled userland experiment with m5ops instrumentation.

Let’s have some fun and try to correlate the gem5 m5out/stats.txt file system.cpu.numCycles cycle count with the x86 RDTSC instruction that is supposed to do the same thing:

./build-userland userland/arch/x86_64/inline_asm/rdtsc.S
./run --eval './arch/x86_64/rdtsc.out;m5 exit;' --emulator gem5
./gem5-stat

RDTSC outputs a cycle count which we compare with gem5’s gem5-stat:

which gives pretty close results, and serve as a nice sanity check that the cycle counter is coherent.

It is also nice to see that RDTSC is a bit smaller than the stats.txt value, since the latter also includes the exec syscall for m5.

Bibliography: