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-rw-r--r--Documentation/kvm/mmu.txt23
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
===============