On Thu, 2009-04-02 at 11:56 -0700, Rahul Kulkarni wrote: > > Rahul, one major quirk we exploit is that Linux does not use the > MSR[AS] > bit at all. One way that bit could be used is to give 32-bit userspace > a > separate 4GB address space from the kernel. Instead, Linux puts both > kernel and userspace into the same 4GB address space (with Linux > mappings above 0xc0000000 and user mappings below). If NetBSD uses > MSR[AS]=1 for userspace (which I think is what the hardware architects > envisioned), you're going to have a lot of MMU fun. > > Rahul>> The NetBSD port for e500/85xx which we have uses the MSR[AS] > (IS/DS) for user/kernel address space separation which keep the > address spaces split. So that's a major problem to start with. How do > we get creative with this to provide guest mappings is something, > which has to be explored. Let me know if you have any thoughts..
OK, so this is going to be a fun one if you like this sort of thing. (I like this sort of thing, but unfortunately don't have any time I can commit to it.) I haven't thought through the details all the way, but at a high level here are my thoughts: First, to understand the architecture and the shortcut we're using today, read http://www.linux-kvm.org/page/PowerPC_Book_E_MMU . Now if you don't have the AS shortcut (which you don't), the key observation is that the guest is really is a collection of 4GB address spaces, and those are identified by 9-bit AS|PID. (By the way, does NetBSD use PID1 and PID2? I sure hope not... :) You can treat the 2^9 guest spaces as separate host spaces. When the guest uses a space, reserve a host space for it, and then map guest AS| PID to the host spaces. So for example: * Guest creates a new process and gives it PID 7. * KVM reserves a new host PID. Let's say host PID 23 is available. * Guest creates a mapping (tlbwe) for PID 7, EA 0xc00000000, RA 0x0. * Host intercepts this (it's a privilege violation because guest is running with MSR[PR]=1). * Host already translates real address from guest physical to host physical. Let's say guest physical 0 corresponds to host physical 128M. * Your new code: host *also* translates guest PID (7) to host PID (23). * Resulting shadow mapping: PID 23, EA 0xc0000000, RA 0x02000000. You'll probably want all shadow mappings to have AS=1. In that case, you would treat guest AS=0 PID=7 as a separate host address space from guest AS=1 PID=7. gAS|gPID 0|7 would be hAS|hPID 1|23, and gAS|gPID 1|7 would be hAS|hPID 1|24. In other words, each guest task (PID) will consume two host address spaces (two different host PIDs, one for each guest AS value). Alexander Graf has already done something like this for his 970 work, so he might be able to provide more details or issues to be aware of in a scheme like this. It would be easier to whiteboard, but obviously that's not really an option... -- Hollis Blanchard IBM Linux Technology Center -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
