Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
> Document it to Documentation/virtual/kvm/mmu.txt
>
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
> Documentation/virtual/kvm/mmu.txt | 23 +++++++++++++++++++++++
> arch/x86/include/asm/kvm_host.h | 5 +++++
> 2 files changed, 28 insertions(+)
>
> diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
> index b5ce7dd..f5c4de9 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:
> + It is the generation number of the page which cooperates with
> + kvm->arch.mmu_valid_gen to fast invalidate all pages.
> + (see "Fast invalidate all pages" below.)
+ 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:
> It is only used on 32bit host which helps us to detect whether updating the
> 64bit spte is complete so that we can avoid reading the truncated value out
> @@ -373,6 +377,25 @@ 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.
>
> +Fast invalidate all pages
> +===========
> +For the large memory and large vcpus guests, zapping all pages is a challenge
> +since they have large number of pages need to be zapped, walking and zapping
> +these pages are really slow and it should hold mmu-lock which stops the memory
> +access on all vcpus.
> +
> +To make it be more scalable, kvm maintains a global mmu valid
> +generation-number which is stored in kvm->arch.mmu_valid_gen and every shadow
> +page stores the current global generation-number into sp->mmu_valid_gen when
> +it is created.
> +
> +When KVM need zap all shadow pages sptes, it just simply increases the global
> +generation-number then reload root shadow pages on all vcpus. Vcpu will create
> +a new shadow page table according to current kvm's generation-number. It
> +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.
> +
+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
> ===============
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 5eb5382..c4f90f6 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -222,6 +222,11 @@ 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 generation number of the page which cooperates with
> + * kvm->arch.mmu_valid_gen to fast invalidate all pages.
> + */
+ /* The page is obsolete if mmu_valid_gen != kvm->arch.mmu_valid_gen. */
Paolo
> unsigned long mmu_valid_gen;
> DECLARE_BITMAP(unsync_child_bitmap, 512);
>
>
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