Messages in this thread | | | Date | Thu, 6 Jan 2022 10:01:39 +0800 | Subject | Re: [RFC PATCH 5/6] KVM: X86: Alloc pae_root shadow page | From | Lai Jiangshan <> |
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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.
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
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.
If you can't try this patchset, the mmu->pae_root can be possible to be modified to test it.
I guess the vmx fails to translate %rip when VMentry in this case.
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