Addressing Meltdown and Spectre in the kernel

The thing you might be missing is that the attack is "only" interesting across privilege domains. In the javascript case that's being able to access all memory from within the JS sandbox. In the userland->kernel case that's accessing all kernel memory, etc. What that basically means that protective measures have to be taken at each of the domain transitions, e.g. when entering/exiting javascript, when performing a syscall, etc. Various patches to e.g. linux have been presented that make userland/kernel transition more secure (by flushing various caches, by preventing dangerous speculation, ...). The problem is that such domain transitions exist in a lot more places...