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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: <87lfxqdp3n.fsf@vitty.brq.redhat.com>
Date:   Tue, 25 Jun 2019 14:01:36 +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: <E7C72E0C-B44F-4CE6-8325-EA32521D75B7@oracle.com>
References: <20190624133028.3710-1-vkuznets@redhat.com>
 <CEFF2A14-611A-417C-BC0A-8814862F26C6@oracle.com>
 <87lfxqdp3n.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 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.

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.

i.e. I thought about having this version of the method:

+u64 nested_enlightened_vmentry(struct kvm_vcpu *vcpu)
+{
+	struct hv_vp_assist_page assist_page;
+
+	if (unlikely(!kvm_hv_get_assist_page(vcpu, &assist_page)))
+		return -1ull;
+
+	if (unlikely(!assist_page.enlighten_vmentry))
+		return -1ull;
+
+	return assist_page.current_nested_vmcs;
+}
+

-Liran

>=20
> If you still dislike nested_enlightened_vmentry() having the side =
effect
> of fetching evmcs_ptr I can get rid of it by splitting the function =
into
> two, however, it will be less efficient for
> nested_vmx_handle_enlightened_vmptrld(). Or we can just leave things =
as
> they are there and use the newly introduced function in =
handle_vmclear()
> only.
>=20
> --=20
> Vitaly