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

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Date:   Fri, 15 Jan 2021 16:08:54 -0800
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>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
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>
> ---
>  arch/x86/kvm/vmx/vmx.c | 15 ++++++---------
>  arch/x86/kvm/vmx/vmx.h |  3 +++
>  arch/x86/kvm/x86.c     |  2 +-
>  3 files changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 819c185adf09..dc778c7b5a06 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -129,6 +129,9 @@ static bool __read_mostly enable_preemption_timer = 1;
>  module_param_named(preemption_timer, enable_preemption_timer, bool, S_IRUGO);
>  #endif
>
> +extern bool __read_mostly allow_smaller_maxphyaddr;

Since this variable is in the kvm module rather than the kvm_intel
module, its current setting is preserved across "rmmod kvm_intel;
modprobe kvm_intel." That is, if set to true, it doesn't revert to
false after "rmmod kvm_intel." Is that the intended behavior?