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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
> Document it to Documentation/virtual/kvm/mmu.txt
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> ---
>  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.

As mentioned in "Reaction to events" above, kvm will cache MMIO
information in leaf sptes.  When a new memslot is added or an existing
memslot is changed, this information may become stale and needs to be
invalidated.  This also needs to hold the MMU lock while walking all
shadow pages, and is made more scalable with a similar technique.

MMIO sptes have a few spare bits, which are used to store a
generation number.  The global generation number is stored in
kvm_memslots(kvm)->generation, and increased whenever guest memory info
changes.  This generation number is distinct from the one described in
the previous section.

When KVM finds an MMIO spte, it checks the generation number of the spte.
If the generation number of the spte does not equal the global generation
number, it will ignore the cached MMIO information and handle the page
fault through the slow path.

Since only 19 bits are used to store generation-number on mmio spte, all
pages are zapped when there is an overflow.


I'm also adding this in the page fault section:

   - check for valid generation number in the spte (see "Fast invalidation of
     MMIO sptes" below)

>  Further reading
>  ===============
>  
>