[PATCH 0/33] KVM: MMU: Cache shadow page tables

From: Avi Kivity
Date: Thu Jan 04 2007 - 10:49:11 EST


The current kvm shadow page table implementation does not cache shadow page tables (except for global translations, used for kernel addresses) across context switches. This means that after a context switch, every memory access will trap into the host. After a while, the shadow page tables will be rebuild, and the guest can proceed at native speed until the next context switch.

The natural solution, then, is to cache shadow page tables across context switches. Unfortunately, this introduces a bucketload of problems:

- the guest does not notify the processor (and hence kvm) that it modifies a page table entry if it has reason to believe that the modification will be followed by a tlb flush. It becomes necessary to write-protect guest page tables so that we can use the page fault when the access occurs as a notification.
- write protecting the guest page tables means we need to keep track of which ptes map those guest page table. We need to add reverse mapping for all mapped writable guest pages.
- when the guest does access the write-protected page, we need to allow it to perform the write in some way. We do that either by emulating the write, or removing all shadow page tables for that page and allowing the write to proceed, depending on circumstances.

This patchset implements the ideas above. While a lot of tuning remains to be done (for example, a sane page replacement algorithm), a guest running with this patchset applied is much faster and more responsive than with 2.6.20-rc3. Some preliminary benchmarks are available in http://article.gmane.org/gmane.comp.emulators.kvm.devel/661.

The patchset is bisectable compile-wise.

--
error compiling committee.c: too many arguments to function

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