Gleb Natapov wrote on 2013-08-06: > On Tue, Aug 06, 2013 at 02:12:51PM +0000, Zhang, Yang Z wrote: >> Gleb Natapov wrote on 2013-08-06: >>> On Tue, Aug 06, 2013 at 11:44:41AM +0000, Zhang, Yang Z wrote: >>>> Gleb Natapov wrote on 2013-08-06: >>>>> On Tue, Aug 06, 2013 at 10:39:59AM +0200, Jan Kiszka wrote: >>>>>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx> >>>>>> >>>>>> If nested EPT is enabled, the L2 guest may change CR3 without any >>>>>> exits. We therefore have to read the current value from the VMCS >>>>>> when switching to L1. However, if paging wasn't enabled, L0 tracks >>>>>> L2's CR3, and GUEST_CR3 rather contains the real-mode identity map. >>>>>> So we need to retrieve CR3 from the architectural state after >>>>>> conditionally updating it - and this is what kvm_read_cr3 does. >>>>>> >>>>> I have a headache from trying to think about it already, but >>>>> shouldn't >>>>> L1 be the one who setups identity map for L2? I traced what >>>>> vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) return here and do not >>>>> see >>>> Here is my understanding: >>>> In vmx_set_cr3(), if enabled ept, it will check whether target >>>> vcpu is enabling >>> paging. When L2 running in real mode, then target vcpu is not >>> enabling paging and it will use L0's identity map for L2. If you >>> read GUEST_CR3 from VMCS, then you may get the L2's identity map >>> not > L1's. >>>> >>> Yes, but why it makes sense to use L0 identity map for L2? I didn't >>> see different vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) values because >>> L0 and L1 use the same identity map address. When I changed identity >>> address L1 configures vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) are >>> indeed different, but the real CR3 L2 uses points to L0 identity map. >>> If I zero L1 identity map page L2 still works. >>> >> If L2 in real mode, then L2PA == L1PA. So L0's identity map also works >> if L2 is in real mode. >> > That not the point. It may work accidentally for kvm on kvm, but what > if other hypervisor plays different tricks and builds different ident map for its guest? Yes, if other hypervisor doesn't build the 1:1 mapping for its guest, it will fail to work. But I cannot imagine what kind of hypervisor will do this and what the purpose is. Anyway, current logic is definitely wrong. It should use L1's identity map instead L0's. Best regards, Yang -- 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
