If you did for(;;) buffer[(i++)%size] = 'y'; then you'd be correct. However, you do i/o. And the 'y's appear at the i/o driver and have to be made visible for

If you did for(;;) buffer[(i++)%size] = 'y'; then you'd be correct. However, you do i/o. And the 'y's appear at the i/o driver and have to be made visible for the device it's driving, which can be anything, including a process on another CPU core in another socket. If they remained in the issuing CPU cache, I fail to see how the destination device could possibly see them. There are some devices which can snoop the cache (like SoC GPUs and other CPU sockets on some archs) but the snooping is much slower than memory bus. Writing to memory is the only way which a) guarantees the data is available elsewhere and b) is the fastest.