Messages in this thread | | | Date | Fri, 7 Jan 2022 12:36:55 +0800 | Subject | Re: [RFC PATCH 5/6] KVM: X86: Alloc pae_root shadow page | From | Lai Jiangshan <> |
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On 2022/1/7 03:41, Sean Christopherson wrote: > On Thu, Jan 06, 2022, Lai Jiangshan wrote: >> >> >> On 2022/1/6 00:45, Sean Christopherson wrote: >>> On Wed, Jan 05, 2022, Lai Jiangshan wrote: >>>> On Wed, Jan 5, 2022 at 5:54 AM Sean Christopherson <seanjc@google.com> wrote: >>>> >>>>>> >>>>>> default_pae_pdpte is needed because the cpu expect PAE pdptes are >>>>>> present when VMenter. >>>>> >>>>> That's incorrect. Neither Intel nor AMD require PDPTEs to be present. Not present >>>>> is perfectly ok, present with reserved bits is what's not allowed. >>>>> >>>>> Intel SDM: >>>>> A VM entry that checks the validity of the PDPTEs uses the same checks that are >>>>> used when CR3 is loaded with MOV to CR3 when PAE paging is in use[7]. If MOV to CR3 >>>>> would cause a general-protection exception due to the PDPTEs that would be loaded >>>>> (e.g., because a reserved bit is set), the VM entry fails. >>>>> >>>>> 7. This implies that (1) bits 11:9 in each PDPTE are ignored; and (2) if bit 0 >>>>> (present) is clear in one of the PDPTEs, bits 63:1 of that PDPTE are ignored. >>>> >>>> But in practice, the VM entry fails if the present bit is not set in the >>>> PDPTE for the linear address being accessed (when EPT enabled at least). The >>>> host kvm complains and dumps the vmcs state. >>> >>> That doesn't make any sense. If EPT is enabled, KVM should never use a pae_root. >>> The vmcs.GUEST_PDPTRn fields are in play, but those shouldn't derive from KVM's >>> shadow page tables. >> >> Oh, I wrote the negative what I want to say again when I try to emphasis >> something after I wrote a sentence and modified it several times. >> >> I wanted to mean "EPT not enabled" when vmx. > > Heh, that makes a lot more sense. > >> The VM entry fails when the guest is in very early stage when booting which >> might be still in real mode. >> >> VMEXIT: intr_info=00000000 errorcode=0000000 ilen=00000000 >> reason=80000021 qualification=0000000000000002 > > Yep, that's the signature for an illegal PDPTE at VM-Enter. But as noted above, > a not-present PDPTE is perfectly legal, VM-Enter should failed if and only if a > PDPTE is present and has reserved bits set. > >> IDTVectoring: info=00000000 errorcode=00000000 >> >>> >>> And I doubt there is a VMX ucode bug at play, as KVM currently uses '0' in its >>> shadow page tables for not-present PDPTEs. >>> >>> If you can post/provide the patches that lead to VM-Fail, I'd be happy to help >>> debug. >> >> If you can try this patchset, you can just set the default_pae_pdpte to 0 to test >> it. > > I can't reproduce the failure with this on top of your series + kvm/queue (commit > cc0e35f9c2d4 ("KVM: SVM: Nullify vcpu_(un)blocking() hooks if AVIC is disabled")). >
I can't reproduce the failure with this code base either. And I can't reproduce the failure when I switch to the code base when I developed it.
After reviewing all the logs I saved that time, I think it was fixed after make_pae_pdpte(). I should have added make_pae_pdpte() first before added default_pae_pdpte. (The code was still mess and the guest can't fully function even when make_pae_pdpte() was added that time)
Removing default_pae_pdpte will simplify the code. Thank you.
Thanks Lai.
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