linux-kernel - Re: [RFC PATCH 5/6] KVM: X86: Alloc pae

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Date:   Thu, 6 Jan 2022 10:01:39 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 5/6] KVM: X86: Alloc pae_root shadow page



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@...gle.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.