I think it's worth noting that in kernel-land most things have fixed maximum sizes anyway. So while a VLA may seem like it's saving time it is probably cheaper

I think it's worth noting that in kernel-land most things have fixed maximum sizes anyway. So while a VLA may seem like it's saving time it is probably cheaper to just use a fixed size array in most situations. In the cases where you really need something of dynamic size I suspect kmalloc isn't that expensive vs the pain that an implicit alloca can have at the wrong time. I'd also guess they probably don't play well with some interrupt handling code as well, as most interrupt handlers can't trigger a page fault or the kernel will panic. It's also worth noting that in general kernel stacks are usually much smaller than userland stacks almost to the point of painful.