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

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Message-ID: <jpgshhscnx5.fsf@linux.bootlegged.copy>
Date:   Wed, 19 Jul 2017 13:54:46 -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:

> 2017-07-17 13:58-0400, Bandan Das:
>> Radim Krčmář <rkrcmar@...hat.com> writes:
>> ...
>>>> > 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.
>>>
>>> AFAIK, we don't support #VE, but AD would be nice to handle from the
>>> beginning.  (I think that caling nested_ept_init_mmu_context() as-is
>>> isn't that bad.)
>> 
>> I went back to the spec to take a look at the AD handling. It doesn't look
>> like anything needs to be done since nested_ept_init_mmu_context() is already
>> being called with the correct eptp in prepare_vmcs02 ? Anything else that
>> needs to be done for AD handling in vmfunc context ?
>
> AD is decided by EPTP bit 6, so it can be toggled by EPTP switching and
> we don't call prepare_vmcs02() after emulating VMFUNC vm exit.
> We want to forward the new AD configuration to KVM's MMU.

Thanks, I had incorrectly assumed that prepare_vmcs02 will be called after
an exit unconditionally. I will work something up soon.

Bandan