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KVM: MMU: document fast invalidate all pages

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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>
This commit is contained in:
Xiao Guangrong 2013-06-19 17:09:24 +08:00 committed by Gleb Natapov
parent 2d49c47f35
commit f6f8adeef5
2 changed files with 28 additions and 0 deletions
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25
Documentation/virtual/kvm/mmu.txt
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@ -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
===============

3
arch/x86/include/asm/kvm_host.h
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@ -222,7 +222,10 @@ struct kvm_mmu_page {
int root_count; /* Currently serving as active root */
unsigned int unsync_children;
unsigned long parent_ptes; /* Reverse mapping for parent_pte */
/* The page is obsolete if mmu_valid_gen != kvm->arch.mmu_valid_gen. */
unsigned long mmu_valid_gen;
DECLARE_BITMAP(unsync_child_bitmap, 512);
#ifdef CONFIG_X86_32