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| -rw-r--r-- | Documentation/kvm/mmu.txt | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/Documentation/kvm/mmu.txt b/Documentation/kvm/mmu.txt index 1e7ecdd15bba..8cb42b957c73 100644 --- a/Documentation/kvm/mmu.txt +++ b/Documentation/kvm/mmu.txt @@ -317,6 +317,29 @@ on fault type: (user write faults generate a #PF) +Large pages +=========== + +The mmu supports all combinations of large and small guest and host pages. +Supported page sizes include 4k, 2M, 4M, and 1G. 4M pages are treated as +two separate 2M pages, on both guest and host, since the mmu always uses PAE +paging. + +To instantiate a large spte, four constraints must be satisfied: + +- the spte must point to a large host page +- the guest pte must be a large pte of at least equivalent size (if tdp is + enabled, there is no guest pte and this condition is satisified) +- if the spte will be writeable, the large page frame may not overlap any + write-protected pages +- the guest page must be wholly contained by a single memory slot + +To check the last two conditions, the mmu maintains a ->write_count set of +arrays for each memory slot and large page size. Every write protected page +causes its write_count to be incremented, thus preventing instantiation of +a large spte. The frames at the end of an unaligned memory slot have +artificically inflated ->write_counts so they can never be instantiated. + Further reading =============== |