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authorXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>2013-06-19 17:09:24 +0800
committerGleb Natapov <gleb@redhat.com>2013-06-27 14:20:47 +0300
commitf6f8adeef542a18b1cb26a0b772c9781a10bb477 (patch)
tree60399cfb430de99e170e68bb2a1c7be596e680c0 /Documentation/virtual
parent2d49c47f350b939b395cd2d1abaa8c3bb6c54326 (diff)
KVM: MMU: document fast invalidate all pages
Document it to Documentation/virtual/kvm/mmu.txt Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virtual')
-rw-r--r--Documentation/virtual/kvm/mmu.txt25
1 files changed, 25 insertions, 0 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index 42193f206602..89c8a4caf51e 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -210,6 +210,10 @@ Shadow pages contain the following information:
A bitmap indicating which sptes in spt point (directly or indirectly) at
pages that may be unsynchronized. Used to quickly locate all unsychronized
pages reachable from a given page.
+ mmu_valid_gen:
+ Generation number of the page. It is compared with kvm->arch.mmu_valid_gen
+ during hash table lookup, and used to skip invalidated shadow pages (see
+ "Zapping all pages" below.)
clear_spte_count:
Only present on 32-bit hosts, where a 64-bit spte cannot be written
atomically. The reader uses this while running out of the MMU lock
@@ -375,6 +379,27 @@ 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
artificially inflated ->write_counts so they can never be instantiated.
+Zapping all pages (page generation count)
+=========================================
+
+For the large memory guests, walking and zapping all pages is really slow
+(because there are a lot of pages), and also blocks memory accesses of
+all VCPUs because it needs to hold the MMU lock.
+
+To make it be more scalable, kvm maintains a global generation number
+which is stored in kvm->arch.mmu_valid_gen. Every shadow page stores
+the current global generation-number into sp->mmu_valid_gen when it
+is created. Pages with a mismatching generation number are "obsolete".
+
+When KVM need zap all shadow pages sptes, it just simply increases the global
+generation-number then reload root shadow pages on all vcpus. As the VCPUs
+create new shadow page tables, the old pages are not used because of the
+mismatching generation number.
+
+KVM then walks through all pages and zaps obsolete pages. While the zap
+operation needs to take the MMU lock, the lock can be released periodically
+so that the VCPUs can make progress.
+
Further reading
===============