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Diffstat (limited to 'Documentation/powerpc/kvm_440.txt')
| -rw-r--r-- | Documentation/powerpc/kvm_440.txt | 41 |
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diff --git a/Documentation/powerpc/kvm_440.txt b/Documentation/powerpc/kvm_440.txt deleted file mode 100644 index c02a003f..0000000 --- a/Documentation/powerpc/kvm_440.txt +++ /dev/null @@ -1,41 +0,0 @@ -Hollis Blanchard <hollisb@us.ibm.com> -15 Apr 2008 - -Various notes on the implementation of KVM for PowerPC 440: - -To enforce isolation, host userspace, guest kernel, and guest userspace all -run at user privilege level. Only the host kernel runs in supervisor mode. -Executing privileged instructions in the guest traps into KVM (in the host -kernel), where we decode and emulate them. Through this technique, unmodified -440 Linux kernels can be run (slowly) as guests. Future performance work will -focus on reducing the overhead and frequency of these traps. - -The usual code flow is started from userspace invoking an "run" ioctl, which -causes KVM to switch into guest context. We use IVPR to hijack the host -interrupt vectors while running the guest, which allows us to direct all -interrupts to kvmppc_handle_interrupt(). At this point, we could either -- handle the interrupt completely (e.g. emulate "mtspr SPRG0"), or -- let the host interrupt handler run (e.g. when the decrementer fires), or -- return to host userspace (e.g. when the guest performs device MMIO) - -Address spaces: We take advantage of the fact that Linux doesn't use the AS=1 -address space (in host or guest), which gives us virtual address space to use -for guest mappings. While the guest is running, the host kernel remains mapped -in AS=0, but the guest can only use AS=1 mappings. - -TLB entries: The TLB entries covering the host linear mapping remain -present while running the guest. This reduces the overhead of lightweight -exits, which are handled by KVM running in the host kernel. We keep three -copies of the TLB: - - guest TLB: contents of the TLB as the guest sees it - - shadow TLB: the TLB that is actually in hardware while guest is running - - host TLB: to restore TLB state when context switching guest -> host -When a TLB miss occurs because a mapping was not present in the shadow TLB, -but was present in the guest TLB, KVM handles the fault without invoking the -guest. Large guest pages are backed by multiple 4KB shadow pages through this -mechanism. - -IO: MMIO and DCR accesses are emulated by userspace. We use virtio for network -and block IO, so those drivers must be enabled in the guest. It's possible -that some qemu device emulation (e.g. e1000 or rtl8139) may also work with -little effort. |
