next up previous
Next: Performance improvements Up: Future work Previous: Protection of kernel memory

Miscellaneous functionality

At this writing, the user-mode port is nearly fully functional. There's a major omission and some minor ones:

SMP Emulation
This is the most serious missing functionality. This will allow a virtual machine to be configured with multiple virtual processors, regardless of how many physical processors are present on the host. This will allow SMP development and testing to happen on uniprocessor machines. It will also enable scaling experiments which test the scalability of the kernel on many more processors than are present on the host. This will possibly allow the kernel to get ahead of the available hardware in terms of processor scalability. SMP emulation is also a way of allocating CPU time to virtual machines. If one virtual machine is given one processor and another is given two, and they're both busy, the two-processor machine will get twice as much CPU time as the uniprocessor machine because it will have two processes running on the host versus the one being run by the uniprocessor machine.
SA_SIGINFO support
The SA_SIGINFO flag allows processes to request extra information about signals that they've received. There is nothing difficult about supporting this option - it just hasn't been needed so far.
Privileged instruction emulation
A number of i386 instructions require that the process executing them have special permission from the processor to run them. The most common examples are the in and out set of instructions and sti and cli. These make no sense to run as-is because there is no hardware for in and out to talk to, and because sti and cli are not how interrupts are enabled and masked. So, they will have to be emulated using the equivalent user-space mechanisms. Very few applications actually use these instructions, which is why this hasn't been implemented yet.
Virtual Ethernet enhancements
The virtual Ethernet daemon can exchange packets with the host and directly with the host's Ethernet, but it also needs to be able to talk to its peers on other machines. This will allow a virtual Ethernet to span multiple physical machines without putting the Ethernet frames directly on the Ethernet, allowing a multi-machine virtual Ethernet to remain isolated from the physical Ethernet.
Currently, the user-mode kernel can not run itself. The inner kernel hangs after getting fifty or sixty system calls into init. To some extent, this is just a stupid kernel trick, but it does have some utility. The main one is testing. It is a demanding process. It exercises many features of the host kernel that aren't used very often. So if it can run itself correctly, that is a good indication that it is functional and correct. The other main use for nesting is testing itself on multiple distributions. There have been bugs and permission problems which show up one some distributions and not others. So, this would be a convenient way of making sure it works on many distributions.

next up previous
Next: Performance improvements Up: Future work Previous: Protection of kernel memory
Jeff Dike 2000-08-25