Posts

MIPS Virtualization: VCPU Address Map

Xvisor sees and works on the actual address map as defined by the MIPS architecure. But for the virtual CPUs (VCPUs) the address space is different. The VCPU running the guest OS sees the KSEG0 (Mapped cached region) starting from 0x00000000. This is where the RAM for the VCPU is mapped. When VCPU is in Kernel Mode or its EXL bit is set, the 2GB region starting from 0x00000000 pretends to be the region enclosed between addresses 0x80000000 - 0xFFFFFFFF on an actual MIPS CPU. When not running in kernel mode, this 2GB region is the regular USEG0. When a VCPU starts, its EXL bit is set and its essentially running in Kernel mode. So USEG0 of CPU is presented as usual kernel mode segments by the hypervisor. Since in this mode, 512 MB region starting at 0x20000000 becomes KSEG1 (mapped, uncached), the VCPU starts running at virtual address 0x3FC00000 (i.e. it becomes the start_pc of VCPU).This is the region marked as "ROM" region under guest in DTS file. The physical address ma

Atomthreads on MIPS architecture

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.

MIPS: non-virtualizable architecture (Part 1)

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

Making of Hyperthreads

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

Null Trace domain registered today

Today I have registered the NullTrace domain. Since I did it with google's help, google app came along. Every thing was a breeze. So you can access this blog at www[dot]nulltrace[dot]org. I *think* I will write some random rants, kernel topics and the posts that won't fit in twitter ;-)