On 06/19/2013 08:35 PM, Paolo Bonzini wrote: > 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 | 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. Really nice. > > > 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) Oh, i forgot it in this section, thanks for your fix. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
