Virtualization: The elegant way and the x86 way

Virtualization means running one or more complete operating systems (at the same time) on one machine, possibly on top of another operating system. VMware, VirtualPC, Parallels etc. support, for example, running a complete GNU/Linux OS on top of Windows. For virtualization, the Virtual Machine Monitor (VMM) must be more powerful than kernel mode code of the guest: The guest’s kernel mode code must not be allowed to change the global state of the machine, but may not notice that its attempts fail, as it was designed for kernel mode. The VMM as the arbiter must be able to control the guest completely. read more

Asking the kernel how to make a syscall

Imagine you’re an i386 user mode application on a modern operating system, and you want to make a syscall, for example to request some memory or create a new thread. But syscalls can be made in various ways on the i386 family of CPUs (int, call gates, sysenter, syscall), and CPUs tend to support only a subset of them. But hardcoding “int” into the kernel is a waste of resources on modern CPUs, because sysenter is a lot faster. read more