Issue with rdcycle Always Returning the Same Value on Linux

[David Novo]

Hi,

When I try to use rdcycle as follows:

asm volatile ("rdcycle %0" : "=r" (cycles));

to profile benchmarks running on Linux, I always get the same number of cycles.

For example, profiling the following code:

uint64_t start, end;

start = read_cycles();

printf("Starting\n");

end = read_cycles();

printf("Cycles taken: %llu from %llu to %llu\n", end - start, start, end);

produces this output:

Cycles taken: 0 from 3046933860 to 3046933860

Could this be related to kernel restrictions or configuration parameters that disable access to performance counters in user space?

Thanks!

[David Novo]

Found the solution here: https://github.com/firesim/FireMarshal/pull/281