linux-kernel - Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1 hypervisor

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Date:   Tue, 11 Jul 2017 15:50:36 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1 hypervisor

Radim Krčmář <rkrcmar@...hat.com> writes:

> 2017-07-11 14:24-0400, Bandan Das:
>> Bandan Das <bsd@...hat.com> writes:
>> > If there's a triple fault, I think it's a good idea to inject it
>> > back. Basically, there's no need to take care of damage control
>> > that L1 is intentionally doing.
>> >
>> >>> +			goto fail;
>> >>> +		kvm_mmu_unload(vcpu);
>> >>> +		vmcs12->ept_pointer = address;
>> >>> +		kvm_mmu_reload(vcpu);
>> >>
>> >> I was thinking about something like this:
>> >>
>> >> kvm_mmu_unload(vcpu);
>> >> old = vmcs12->ept_pointer;
>> >> vmcs12->ept_pointer = address;
>> >> if (kvm_mmu_reload(vcpu)) {
>> >> 	/* pointer invalid, restore previous state */
>> >> 	kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>> >> 	vmcs12->ept_pointer = old;
>> >> 	kvm_mmu_reload(vcpu);
>> >> 	goto fail;
>> >> }
>> >>
>> >> The you can inherit the checks from mmu_check_root().
>> 
>> Actually, thinking about this a bit more, I agree with you. Any fault
>> with a vmfunc operation should end with a vmfunc vmexit, so this
>> is a good thing to have. Thank you for this idea! :)
>
> SDM says
>
>   IF tent_EPTP is not a valid EPTP value (would cause VM entry to fail
>   if in EPTP) THEN VMexit;

This section here:
As noted in Section 25.5.5.2, an execution of the
EPTP-switching VM function that causes a VM exit (as specified
above), uses the basic exit reason 59, indicating “VMFUNC”.
The length of the VMFUNC instruction is saved into the
VM-exit instruction-length field. No additional VM-exit
information is provided.

Although, it adds (as specified above), from testing, any vmexit that
happens as a result of the execution of the vmfunc instruction always
has exit reason 59.

IMO, the case David pointed out comes under "as a result of the
execution of the vmfunc instruction", so I would prefer exiting
with reason 59.

> and no other mentions of a VM exit, so I think that the VM exit happens
> only under these conditions:
>
>   — The EPT memory type (bits 2:0) must be a value supported by the
>     processor as indicated in the IA32_VMX_EPT_VPID_CAP MSR (see
>     Appendix A.10).
>   — Bits 5:3 (1 less than the EPT page-walk length) must be 3, indicating
>     an EPT page-walk length of 4; see Section 28.2.2.
>   — Bit 6 (enable bit for accessed and dirty flags for EPT) must be 0 if
>     bit 21 of the IA32_VMX_EPT_VPID_CAP MSR (see Appendix A.10) is read
>     as 0, indicating that the processor does not support accessed and
>     dirty flags for EPT.
>   — Reserved bits 11:7 and 63:N (where N is the processor’s
>     physical-address width) must all be 0.
>
> And it looks like we need parts of nested_ept_init_mmu_context() to
> properly handle VMX_EPT_AD_ENABLE_BIT.

I completely ignored AD and the #VE sections. I will add a TODO item
in the comment section.

> The KVM_REQ_TRIPLE_FAULT can be handled by kvm_mmu_reload in vcpu_run if
> we just invalidate the MMU.