Belt and suspenders. In principle you don't need to protect against spectre and meltdown if you can ensure that no untrusted code runs on a particular machine.

Belt and suspenders. In principle you don't need to protect against spectre and meltdown if you can ensure that no untrusted code runs on a particular machine. How do you ensure that though? There are many many routes for that to happen, and it only has to happen once for it to completely invalidate your security model if you have no other levels of protection. On a server you can run untrusted code by exploiting other vulnerabilities in server software such as buffer overruns. It's unrealistic to expect that all servers will have no such vulnerabilities in any of their software at any time. That's why operating systems implement many layers of protections to prevent those exploits from being elevated into even worse breaches that could leak sensitive data, provide local root access, and so on. One of the more significant protections that exists today is address space layout randomization, which makes it much more difficult for a buffer overflow exploit to result in immediate privilege escalation. Meltdown/spectre make these sorts of issues much, much worse. They mean that absolute any vulnerability in any service could be exploited to read the entire contents of memory and deliver that to an attacker, including encryption keys, including passwords, and so forth. That's bad. You can essentially 100% guarantee that there will be some vulnerability in some service which enables a minor exploit, and you want to avoid having that minor issue become catastrophic through meltdown/spectre based attacks.