No, the AVX-512 instructions weren’t free. You had to disable all of the E cores. It only made sense if you could afford to give up the extra cores to accelerate a few instructions in a smaller number of threads.
It wasn’t a net win for the average person. I doubt many people would ever do this.
E cores are only present on the high-end Alder Lake SKUs. If you have a mid-range chip with only P cores like the i5-12400, enabling AVX-512 really is free.
Such P-core-only chips are attractive to Linux users. Intel Thread Director, their Alder Lake schedule software, does not work as well on Linux as it does on Windows. It has a tendency to schedule processes on the E cores even when P cores are available, which is bad for performance.
There is significant overlap between users interested in AVX-512 and users interested in Linux performance, making the mid-range Alder Lake chips especially appealing to this group. Unfortunately they top out at 6 cores with no ECC, so it's not a perfect match.
