Atomthread's port to MIPS architecture is complete. A few of the test cases are working fine. Some of them need to be fixed. Working on them. The source can be pulled from here . Its upto Kelvin now when he wants to pull in the changes. Since this is new port to a 32-bit architecture, there are some changes in the core kernel code itself. Hopefully, MIPS will get merged soon. Once I am done with validating and fixing all the test cases, I can go back to the reason I started this port -- my hypervisor. I have not been working on it for one and a half week now. Last one and a half week, I was busy with porting atomthreads to MIPS. I started this port because for testing the hypervisor, I wanted a small OS that doesn't play with MMU much. My first target is to test the instruction emulation framework for which Linux would be too heavy. Hope I will get back to hypervisor soon.
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MIPS is a wonderful architecture. But when it was design virtualization wasn't much in the air. As a result, this architecture isn't compleletly virtualizable. It isn't as notorious as x86. One requirement for virtualization is the access priviledges to syste registers. MIPS does provide this. All system registers are in CP0. Any access from user space will falt. Modification access to TLBs will also fault. Then what is the problem? The problem is with the way virtual address space is laid out. For 32 bit architecture 4 GB virtual address space is available. MIPS specification reserve upper 2 GB for kernel/supervisor mode. Any access to 3rd GB in kernel mode will go untranslated by MMU. The last 4th GB will go via MMU but is only usable in kernel mode. For running a guest, only first 2 GB is available. Both guest and its userspace programs can only have 2GB of virtual address space. Linux assumes many things about address space layout. It is linked in kseg0. Kmap a
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I have been working on a fun project where I am trying to virtualize a MIPS machine. For a QEMU emulated NE2000 based network device and for couple of system daemons, I needed a light threaded framework. While the earlier design was that anything in hypervisor context was run on CPU and thus was completely serialized. So we decided on doing something differently here. We created a virtual CPU and named it hypercore. This virtual CPU is supposed to run anything in context of hypervisor. This virtual CPU, whenever scheduled, always works in kernel mode. The idea was simple, anything of _maintenance_ sort was scheduled later on that hypercore. I then went on implementing the threading mechanism. Its very similar to what is know as kernel threads in Linux(R) world. There is one function which the thread is supposed to execute. Threads have a lifetime: they are created, run, paused, run again and then finally destroyed. Initial implementation was non-preemtible. This means that threa