Non virtualizable instructions
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If the VMM can virtualize some instructions, it could be very bad for the guest. We need to develop a list of instructions that the VMM could use to its advantage and test for them under our virtualization Paravirtualization - "involves modifying the OS kernel to replace non-virtualizable instructions with hypercalls that communicate directly with the virtualization layer hypervisor. ² Non-virtualizable Instruction: those instructions existed in some processor architectures, which are either sensitive or behave differently under different privilege level, but both don't incur traps. Three alternatives for handling non-virtualizable instructions Binary translation Hardware assist OS assist or paravirtualization. Compatibility Performance VMM sophistication. Based on non-virtualizable microprocessors by william D. armitage. to effectively implement a privileged instruction set. The. architecture is shown to be virtualizable Paravirtualization, as shown in Figure 6, involves modifying the OS kernel to replace non-virtualizable instructions with hypercalls that communicate directly with the virtualization layer hypervisor. • Ex: IO instructions • Virtualizable processor: all sensitive instructions are privileged. • If a processor is virtualizable, a VMM can interpose on any sensitive instruction that the VM tries to We call "Sensitive but Non-Privilege instruction" as "Critical Instruction". Non-Virtualizable CPU • Some of sensitive instructions are privilege instructions. The x86 architecture originally contained instructions that were non-virtualizable using trap-and-emulate virtualization. Shadow page tables store the mappings from guest physical to host physical In paravirtualization, these instructions are handled at compile time when the non-virtualizable OS instructions are replaced with hypercalls. (Virtual) memory. A subset of the instruction set of the underlying machine. Most (but not all) of the VMM analyzes all code executed in virtual machine. Replaces non-virtualizable instructions with
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