linux-kernel - Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST

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Date:   Fri, 27 Sep 2019 07:27:25 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reto Buerki <reet@...elabs.ch>
Subject: Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during
 nested VM-Enter

On Fri, Sep 27, 2019 at 03:06:02AM +0300, Liran Alon wrote:
> 
> 
> > On 27 Sep 2019, at 0:43, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> > 
> > 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 “If an emulated VMEntry is squashed because L1 sets
> vmcs12->guest_activity_state to HLT”.  I think it’s a bit more explicit.
> 
> > 
> > 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.
> > 
> > Cc: stable@...r.kernel.org
> > Reported-by: Reto Buerki <reet@...elabs.ch>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > ---
> > arch/x86/kvm/vmx/nested.c | 8 ++++++++
> > arch/x86/kvm/vmx/vmx.c    | 9 ++++++---
> > 2 files changed, 14 insertions(+), 3 deletions(-)
> > 
> > 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;
> > 
> > +	/*
> > +	 * 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’s not exactly if L2 is entering HLT state in
> general.  (E.g. issue doesn’t occur if L2 runs HLT directly which is not
> configured to be intercepted by vmcs12).  It’s specifically when L1 enters L2
> with a HLT guest-activity-state. I suggest rephrasing comment.

I deliberately worded the comment so that it remains valid if there are
more conditions in the future that cause KVM to skip running L2.  What if
I split the difference and make the changelog more explicit, but leave the
comment as is?

> > +	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 = vcpu->kvm;
> > 	unsigned long guest_cr3;
> > +	bool skip_cr3 = false;
> > 	u64 eptp;
> > 
> > 	guest_cr3 = 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);
> > 		}
> > 
> > -		if (enable_unrestricted_guest || is_paging(vcpu) ||
> > -		    is_guest_mode(vcpu))
> > +		if (is_guest_mode(vcpu))
> > +			skip_cr3 = true;
> > +		else if (enable_unrestricted_guest || is_paging(vcpu))
> > 			guest_cr3 = kvm_read_cr3(vcpu);
> > 		else
> > 			guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
> > 		ept_load_pdptrs(vcpu);
> > 	}
> > 
> > -	vmcs_writel(GUEST_CR3, guest_cr3);
> > +	if (!skip_cr3)
> 
> Nit: It’s a matter of taste, but I prefer positive conditions. i.e. “bool
> write_guest_cr3”.
> 
> Anyway, code seems valid to me. Nice catch.
> Reviewed-by: Liran Alon <liran.alon@...cle.com>
> 
> -Liran
> 
> > +		vmcs_writel(GUEST_CR3, guest_cr3);
> > }
> > 
> > int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > -- 
> > 2.22.0
> > 
>