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SubjectRe: [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@intel.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@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(-)
> >
> > 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@oracle.com>
>
> -Liran
>
> > + vmcs_writel(GUEST_CR3, guest_cr3);
> > }
> >
> > int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > --
> > 2.22.0
> >
>

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