linux-kernel - Re: [PATCH v5 09/12] x86/kvm/nVMX: allow bare VMXON state migration

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Date:   Fri, 14 Sep 2018 09:49:32 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Liran Alon <liran.alon@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 09/12] x86/kvm/nVMX: allow bare VMXON state migration

Jim Mattson <jmattson@...gle.com> writes:

> On Thu, Sep 13, 2018 at 10:05 AM, Vitaly Kuznetsov <vkuznets@...hat.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@...hat.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;

Time to restore the status quo :-)

In any case this early exit should be done after enter_vmx_operation()
as even without a valid VMCS we need to preserve VMXON state (e.g. Linux
with loaded KVM but without any guests running).

-- 
  Vitaly