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Subject: Re: [PATCH] x86/kvm/nVMCS: fix VMCLEAR when Enlightened VMCS is in
 use
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <87ftnxex1g.fsf@vitty.brq.redhat.com>
Date:   Tue, 25 Jun 2019 14:18:05 +0300
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Message-Id: <702DE49A-0ED0-4012-B702-F4759B11B1AE@oracle.com>
References: <20190624133028.3710-1-vkuznets@redhat.com>
 <CEFF2A14-611A-417C-BC0A-8814862F26C6@oracle.com>
 <87lfxqdp3n.fsf@vitty.brq.redhat.com>
 <E7C72E0C-B44F-4CE6-8325-EA32521D75B7@oracle.com>
 <87ftnxex1g.fsf@vitty.brq.redhat.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 25 Jun 2019, at 14:15, Vitaly Kuznetsov <vkuznets@redhat.com> =
wrote:
>=20
> Liran Alon <liran.alon@oracle.com> writes:
>=20
>>> On 25 Jun 2019, at 11:51, Vitaly Kuznetsov <vkuznets@redhat.com> =
wrote:
>>>=20
>>> Liran Alon <liran.alon@oracle.com> writes:
>>>=20
>>>>> On 24 Jun 2019, at 16:30, Vitaly Kuznetsov <vkuznets@redhat.com> =
wrote:
>>>>>=20
>>>>>=20
>>>>> +bool nested_enlightened_vmentry(struct kvm_vcpu *vcpu, u64 =
*evmptr)
>>>>=20
>>>> I prefer to rename evmptr to evmcs_ptr. I think it=E2=80=99s more =
readable and sufficiently short.
>>>> In addition, I think you should return either -1ull or =
assist_page.current_nested_vmcs.
>>>> i.e. Don=E2=80=99t return evmcs_ptr by pointer but instead as a =
return-value
>>>> and get rid of the bool.
>>>=20
>>> Actually no, sorry, I'm having second thoughts here: in =
handle_vmclear()
>>> we don't care about the value of evmcs_ptr, we only want to check =
that
>>> enlightened vmentry bit is enabled in assist page. If we switch to
>>> checking evmcs_ptr against '-1', for example, we will make '-1' a =
magic
>>> value which is not in the TLFS. Windows may decide to use it for
>>> something else - and we will get a hard-to-debug bug again.
>>=20
>> I=E2=80=99m not sure I understand.
>> You are worried that when guest have setup a valid assist-page and =
set
>> enlighten_vmentry to true,
>> that assist_page.current_nested_vmcs can be -1ull and still be =
considered a valid eVMCS?
>> I don't think that's reasonable.
>=20
> No, -1ull is not a valid eVMCS - but this shouldn't change VMCLEAR
> semantics as VMCLEAR has it's own argument. It's perfectly valid to =
try
> to put a eVMCS which was previously used on a different vCPU (and thus
> which is 'active') to non-active state. The fact that we don't have an
> active eVMCS on the vCPU doing VMCLEAR shouldn't matter at all.
>=20
> --=20
> Vitaly

Oh oops sure. Yes you are right.
I forgot about the larger context here for a moment.
Sorry for the confusion. :)

-Liran