[PATCH 7/7] KVM: MMU: document fast invalidate all mmio sptes

[Xiao Guangrong]

Document it to Documentation/virtual/kvm/mmu.txt

Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
---
 Documentation/virtual/kvm/mmu.txt | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index f5c4de9..9b7cfb3 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -396,6 +396,31 @@ ensures the old pages are not used any more. The invalid-gen pages
 (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen) are zapped by using lock-break
 technique.
 
+Fast invalidate all mmio sptes
+===========
+As mentioned in "Reaction to events" above, kvm will cache the mmio information
+to the last sptes so that we should zap all mmio sptes when the guest mmio info
+is changed. This will happen when a new memslot is added or the existing
+memslot is moved.
+
+Zapping mmio spte is also a scalability issue for the large memory and large
+vcpus guests since it needs to hold hot mmu-lock and walk all shadow pages to
+find all the mmio spte out.
+
+We fix this issue by using the similar way of "Fast invalidate all pages".
+The global mmio valid generation-number is stored in kvm->memslots.generation
+and every mmio spte stores the current global generation-number into his
+available bits when it is created
+
+The global mmio valid generation-number is increased whenever the gust memory
+info is changed. When guests do mmio access, kvm intercepts a MMIO #PF then it
+walks the shadow page table and get the mmio spte. If the generation-number on
+the spte does not equal the global generation-number, it will go to the normal
+#PF handler to update the mmio spte.
+
+Since 19 bits are used to store generation-number on mmio spte, we zap all pages
+when the number is round.
+
 Further reading
 ===============
 
-- 
1.8.1.4

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