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Subject: Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during
 nested VM-Enter
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <20190926214302.21990-2-sean.j.christopherson@intel.com>
Date:   Fri, 27 Sep 2019 03:06:02 +0300
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Reto Buerki <reet@codelabs.ch>
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Message-Id: <68340081-0094-4A74-9B33-3431F39659AA@oracle.com>
References: <20190926214302.21990-1-sean.j.christopherson@intel.com>
 <20190926214302.21990-2-sean.j.christopherson@intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 27 Sep 2019, at 0:43, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>=20
> Write the desired L2 CR3 into vmcs02.GUEST_CR3 during nested VM-Enter
> isntead of deferring the VMWRITE until vmx_set_cr3().  If the VMWRITE
> is deferred, then KVM can consume a stale vmcs02.GUEST_CR3 when it
> refreshes vmcs12->guest_cr3 during nested_vmx_vmexit() if the emulated
> VM-Exit occurs without actually entering L2, e.g. if the nested run
> is squashed because L2 is being put into HLT.

I would rephrase to =E2=80=9CIf an emulated VMEntry is squashed because =
L1 sets vmcs12->guest_activity_state to HLT=E2=80=9D.
I think it=E2=80=99s a bit more explicit.

>=20
> In an ideal world where EPT *requires* unrestricted guest (and vice
> versa), VMX could handle CR3 similar to how it handles RSP and RIP,
> e.g. mark CR3 dirty and conditionally load it at vmx_vcpu_run().  But
> the unrestricted guest silliness complicates the dirty tracking logic
> to the point that explicitly handling vmcs02.GUEST_CR3 during nested
> VM-Enter is a simpler overall implementation.
>=20
> Cc: stable@vger.kernel.org
> Reported-by: Reto Buerki <reet@codelabs.ch>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> arch/x86/kvm/vmx/nested.c | 8 ++++++++
> arch/x86/kvm/vmx/vmx.c    | 9 ++++++---
> 2 files changed, 14 insertions(+), 3 deletions(-)
>=20
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 41abc62c9a8a..971a24134081 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2418,6 +2418,14 @@ static int prepare_vmcs02(struct kvm_vcpu =
*vcpu, struct vmcs12 *vmcs12,
> 				entry_failure_code))
> 		return -EINVAL;
>=20
> +	/*
> +	 * Immediately write vmcs02.GUEST_CR3.  It will be propagated to =
vmcs12
> +	 * on nested VM-Exit, which can occur without actually running =
L2, e.g.
> +	 * if L2 is entering HLT state, and thus without hitting =
vmx_set_cr3().
> +	 */

If I understand correctly, it=E2=80=99s not exactly if L2 is entering =
HLT state in general.
(E.g. issue doesn=E2=80=99t occur if L2 runs HLT directly which is not =
configured to be intercepted by vmcs12).
It=E2=80=99s specifically when L1 enters L2 with a HLT =
guest-activity-state. I suggest rephrasing comment.

> +	if (enable_ept)
> +		vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
> +
> 	/* Late preparation of GUEST_PDPTRs now that EFER and CRs are =
set. */
> 	if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
> 	    is_pae_paging(vcpu)) {
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d4575ffb3cec..b530950a9c2b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2985,6 +2985,7 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned =
long cr3)
> {
> 	struct kvm *kvm =3D vcpu->kvm;
> 	unsigned long guest_cr3;
> +	bool skip_cr3 =3D false;
> 	u64 eptp;
>=20
> 	guest_cr3 =3D cr3;
> @@ -3000,15 +3001,17 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, =
unsigned long cr3)
> 			spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
> 		}
>=20
> -		if (enable_unrestricted_guest || is_paging(vcpu) ||
> -		    is_guest_mode(vcpu))
> +		if (is_guest_mode(vcpu))
> +			skip_cr3 =3D true;
> +		else if (enable_unrestricted_guest || is_paging(vcpu))
> 			guest_cr3 =3D kvm_read_cr3(vcpu);
> 		else
> 			guest_cr3 =3D =
to_kvm_vmx(kvm)->ept_identity_map_addr;
> 		ept_load_pdptrs(vcpu);
> 	}
>=20
> -	vmcs_writel(GUEST_CR3, guest_cr3);
> +	if (!skip_cr3)

Nit: It=E2=80=99s a matter of taste, but I prefer positive conditions. =
i.e. =E2=80=9Cbool write_guest_cr3=E2=80=9D.

Anyway, code seems valid to me. Nice catch.
Reviewed-by: Liran Alon <liran.alon@oracle.com>

-Liran

> +		vmcs_writel(GUEST_CR3, guest_cr3);
> }
>=20
> int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> --=20
> 2.22.0
>=20