The point of virtualization

The point of virtualization

Posted Oct 13, 2007 2:16 UTC (Sat) by giraffedata (subscriber, #1954)
In reply to: The point of virtualization by roelofs
Parent article: Matt Domsch on Linux support at Dell

The guest app triggers a TLB (cache) miss, which means the guest kernel has to do a virtual-to-"physical" lookup in the page table,

How would the guest kernel get involved? Page translation is done in the CPU. The guest app does a load instruction to an address he hasn't accessed in a while, so its translation is not in the TLB, so the CPU walks the shadow page tables to find to which physical page it refers, then loads the bytes into the register from there, then proceeds to the next instruction. The CPU doesn't even know the guest page table exists; the shadow page table maps the guest ap's virtual addresses to physical memory.

You seem to be saying TLB miss, but thinking page fault. When there's a page fault (the shadow page table does not contain a translation for the address in question), there is that multi-level lookup by the guest kernel and hypervisor, to ultimately add a translation to the shadow page tables. On systems that have guest page tables, that is.

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