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

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Message-ID: <jpg8tjuydax.fsf@linux.bootlegged.copy>
Date:   Tue, 11 Jul 2017 15:38:14 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <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 14:35-0400, Bandan Das:
>> Jim Mattson <jmattson@...gle.com> writes:
>> ...
>> >>> I can find the definition for an vmexit in case of index >=
>> >>> VMFUNC_EPTP_ENTRIES, but not for !vmcs12->eptp_list_address in the SDM.
>> >>>
>> >>> Can you give me a hint?
>> >>
>> >> I don't think there is. Since, we are basically emulating eptp switching
>> >> for L2, this is a good check to have.
>> >
>> > There is nothing wrong with a hypervisor using physical page 0 for
>> > whatever purpose it likes, including an EPTP list.
>> 
>> Right, but of all the things, a l1 hypervisor wanting page 0 for a eptp list
>> address most likely means it forgot to initialize it. Whatever damage it does will
>> still end up with vmfunc vmexit anyway.
>
> Most likely, but not certainly.  I also don't see a to diverge from the
> spec here.

Actually, this is a specific case where I would like to diverge from the spec.
But then again, it's L1 shooting itself in the foot and this would be a rarely
used code path, so, I am fine removing it.