Good catch, thanks! Since 5.14 (I think?) the kernel only reads eax (before that it used all of rax and returned ENOSYS if the upper half wasn't zero), so I'm adding a note about it to the x64 and x32 conventions

According to [1], the kernel has flip-flopped a few times on this, but the original behavior was to ignore the upper 32 bits.

[1] https://lore.kernel.org/all/20210518191303.4135296-5-hpa@zyt...