Guests in a virtualized environment expect a contiguous,
isolated physical address space. To provide this, the hypervisor
translates guest physical addresses into host physical addresses, much
like virtual addresses are translated into physical addresses in an
operating system. There are two approaches to managing these
translations. With shadow page tables, the hypervisor traps on guest
page table modifications and updates the shadow page table, a list of
GVA->HPA translations. This requires no hardware support but involves
many costly VM exits. Nested page tables are an alternative where
both the guest and host page tables are hardware walked. This
eliminates the necessity of trapping into the hypervisor when the
guest page table is updated, but for n-level page tables can result in
O(n*2) accesses to translate each address. I will present a survey of
recent work to speed up address translation, including 2D translation
caching, addition of a nested TLB, and using a hashed page table for
the GPA->HPA translation.