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From:   Jim Mattson <jmattson@google.com>
Date:   Thu, 13 Sep 2018 12:21:13 -0700
Message-ID: <CALMp9eTTK67npZLjpCYA=6EPpvF8nZZq7fH=e7NQRhDYXZ=UNQ@mail.gmail.com>
Subject: Re: [PATCH v5 09/12] x86/kvm/nVMX: allow bare VMXON state migration
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Roman Kagan <rkagan@virtuozzo.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
        Liran Alon <liran.alon@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Sep 13, 2018 at 10:05 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> It is perfectly valid for a guest to do VMXON and not do VMPTRLD. This
> state needs to be preserved on migration.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d3297fadf7ed..25a25fff8dd9 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -14482,13 +14482,6 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>         if (!page_address_valid(vcpu, kvm_state->vmx.vmxon_pa))
>                 return -EINVAL;
>
> -       if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12))
> -               return -EINVAL;
> -
> -       if (kvm_state->vmx.vmcs_pa == kvm_state->vmx.vmxon_pa ||
> -           !page_address_valid(vcpu, kvm_state->vmx.vmcs_pa))
> -               return -EINVAL;
> -
>         if ((kvm_state->vmx.smm.flags & KVM_STATE_NESTED_SMM_GUEST_MODE) &&
>             (kvm_state->flags & KVM_STATE_NESTED_GUEST_MODE))
>                 return -EINVAL;
> @@ -14510,6 +14503,14 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
>         if (ret)
>                 return ret;
>
> +       /* Empty 'VMXON' state is permitted */
> +       if (kvm_state->size < sizeof(kvm_state) + sizeof(*vmcs12))
> +               return 0;

In the original version of this code, the early exit is:

if (kvm_state.vmcs_pa == -1ull)
        return 0;