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Diffstat (limited to 'Documentation/powerpc/kvm_440.txt')
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diff --git a/Documentation/powerpc/kvm_440.txt b/Documentation/powerpc/kvm_440.txt new file mode 100644 index 000000000000..c02a003fa03a --- /dev/null +++ b/Documentation/powerpc/kvm_440.txt @@ -0,0 +1,41 @@ +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. |