linux-kernel - Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space support user-configurable

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Message-ID: <CALMp9eTrc8_z3pKBtLVmbnMvC+KtzXMYbYTXZPPz5F0UWW8oNQ@mail.gmail.com>
Date:   Thu, 3 Sep 2020 10:57:58 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Mohammed Gamal <mgamal@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: x86: VMX: Make smaller physical guest address space
 support user-configurable

On Thu, Sep 3, 2020 at 7:12 AM Mohammed Gamal <mgamal@...hat.com> wrote:
>
> This patch exposes allow_smaller_maxphyaddr to the user as a module parameter.
>
> Since smaller physical address spaces are only supported on VMX, the parameter
> is only exposed in the kvm_intel module.
> Modifications to VMX page fault and EPT violation handling will depend on whether
> that parameter is enabled.
>
> Also disable support by default, and let the user decide if they want to enable
> it.
>
> Signed-off-by: Mohammed Gamal <mgamal@...hat.com>

I think a smaller guest physical address width *should* be allowed.
However, perhaps the pedantic adherence to the architectural
specification could be turned on or off per-VM? And, if we're going to
be pedantic, I think we should go all the way and get MOV-to-CR3
correct.

Does the typical guest care about whether or not setting any of the
bits 51:46 in a PFN results in a fault?