CPU特权环：为什么不使用环1和2？ [英] CPU Privilege Rings: Why rings 1 and 2 aren't used?

问题描述

关于x86 CPU特权环的几个问题：

• 为什么大多数人都不使用环1和2操作系统？仅仅是为了保持与其他体系结构的代码兼容性，还是有更好的理由？




• 是否有任何实际使用这些环的操作系统？还是完全不使用它们？

解决方案

作为一名业余操作系统作家，我发现，由于分页（现代保护模型的主要部分）仅具有特权（0,1,2环）和非特权的概念，因此对环1和2的好处大大减少了。

Intel的第1环和第2环的意图是使操作系统将设备驱动程序置于该级别，因此它们具有特权，但与内核代码的其余部分有所区别。

第1环和第2环具有大部分特权。他们可以访问主管页面，但是如果尝试使用特权指令，它们仍会像环3一样使用GPF。因此，对于英特尔计划的驱动程序来说，这不是一个坏地方。

话虽如此，但它们确实在某些设计中有用。实际上，并非总是直接由OS。例如， VirtualBox ，虚拟机，将来宾内核代码置于环1中。我还可以确定某些操作系统确实使用了它们，我只是暂时认为它不是一种流行的设计。 p>

A couple of questions regarding the x86 CPU privilege rings:

• Why aren't rings 1 and 2 used by most operating systems? Is it just to maintain code compatibility with other architectures, or is there a better reason?

• Are there any operating systems which actually use those rings? Or are they completely unused?

解决方案

As a hobbyist operating system writer, I found that because paging (a major part of the modern protection model) only has a concept of privileged (ring 0,1,2) and unprivileged, the benefit to rings 1 and 2 were diminished greatly.

The intent by Intel in having rings 1 and 2 is for the OS to put device drivers at that level, so they are privileged, but somewhat separated from the rest of the kernel code.

Rings 1 and 2 are in a way, "mostly" privileged. They can access supervisor pages, but if they attempt to use a privileged instruction, they still GPF like ring 3 would. So it is not a bad place for drivers as Intel planned...

That said, they definitely do have use in some designs. In fact, not always directly by the OS. For example, VirtualBox, a Virtual Machine, puts the guest kernel code in ring 1. I am also sure some operating systems do make use of them, I just don't think it is a popular design at the moment.

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