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 16:45:27 -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 15:38-0400, Bandan Das:
>> 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.
>
> Thanks, we're not here to judge the guest, but to provide a bare-metal
> experience. :)

There are certain cases where do. For example, when L2 instruction emulation
fails we decide to kill L2 instead of injecting the error to L1 and let it handle
that. Anyway, that's a different topic, I was just trying to point out there
are cases kvm does a somewhat policy decision...