[PATCH 15/15] KVM: nVMX: Copy PDPTRs to/from vmcs12 only when necessary

Sean Christopherson
2019-05-07T16:06:54Z

Per Intel's SDM:

... the logical processor uses PAE paging if CR0.PG=1, CR4.PAE=1 and
IA32_EFER.LME=0. A VM entry to a guest that uses PAE paging loads the
PDPTEs into internal, non-architectural registers based on the setting
of the "enable EPT" VM-execution control.

and:

[GUEST_PDPTR] values are saved into the four PDPTE fields as follows:

- If the "enable EPT" VM-execution control is 0 or the logical
processor was not using PAE paging at the time of the VM exit,
the values saved are undefined.

In other words, if EPT is disabled or the guest isn't using PAE paging,
then the PDPTRS aren't consumed by hardware on VM-Entry and are loaded
with junk on VM-Exit. From a nesting perspective, all of the above hold
true, i.e. KVM can effectively ignore the VMCS PDPTRs. E.g. KVM already
loads the PDPTRs from memory when nested EPT is disabled (see
nested_vmx_load_cr3()).

Because KVM intercepts setting CR4.PAE, there is no danger of consuming
a stale value or crushing L1's VMWRITEs regardless of whether L1
intercepts CR4.PAE. The vmcs12's values are unchanged up until the
VM-Exit where L2 sets CR4.PAE, i.e. L0 will see the new PAE state on the
subsequent VM-Entry and propagate the PDPTRs from vmcs12 to vmcs02.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/x86/kvm/vmx/nested.c | 36 +++++++++++++++++++++---------------
1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index cfdc04fde8eb..b8bd446b2c8b 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2184,17 +2184,6 @@ static void prepare_vmcs02_full(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
vmcs_writel(GUEST_SYSENTER_ESP, vmcs12->guest_sysenter_esp);
vmcs_writel(GUEST_SYSENTER_EIP, vmcs12->guest_sysenter_eip);

- /*
- * L1 may access the L2's PDPTR, so save them to construct
- * vmcs12
- */
- if (enable_ept) {
- vmcs_write64(GUEST_PDPTR0, vmcs12->guest_pdptr0);
- vmcs_write64(GUEST_PDPTR1, vmcs12->guest_pdptr1);
- vmcs_write64(GUEST_PDPTR2, vmcs12->guest_pdptr2);
- vmcs_write64(GUEST_PDPTR3, vmcs12->guest_pdptr3);
- }
-
if (vmx->nested.nested_run_pending &&
(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
vmcs_write64(GUEST_IA32_DEBUGCTL,
@@ -2256,10 +2245,15 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
struct hv_enlightened_vmcs *hv_evmcs = vmx->nested.hv_evmcs;
+ bool load_guest_pdptrs_vmcs12 = false;

if (vmx->nested.dirty_vmcs12 || vmx->nested.hv_evmcs) {
prepare_vmcs02_full(vmx, vmcs12);
vmx->nested.dirty_vmcs12 = false;
+
+ load_guest_pdptrs_vmcs12 = !vmx->nested.hv_evmcs ||
+ !(vmx->nested.hv_evmcs->hv_clean_fields &
+ HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP1);
}

/*
@@ -2366,6 +2360,15 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
return -EINVAL;
}

+ /* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
+ if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
+ !is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu)) {
+ vmcs_write64(GUEST_PDPTR0, vmcs12->guest_pdptr0);
+ vmcs_write64(GUEST_PDPTR1, vmcs12->guest_pdptr1);
+ vmcs_write64(GUEST_PDPTR2, vmcs12->guest_pdptr2);
+ vmcs_write64(GUEST_PDPTR3, vmcs12->guest_pdptr3);
+ }
+
/* Shadow page tables on either EPT or shadow page tables. */
if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
entry_failure_code))
@@ -3467,10 +3470,13 @@ static void sync_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
*/
if (enable_ept) {
vmcs12->guest_cr3 = vmcs_readl(GUEST_CR3);
- vmcs12->guest_pdptr0 = vmcs_read64(GUEST_PDPTR0);
- vmcs12->guest_pdptr1 = vmcs_read64(GUEST_PDPTR1);
- vmcs12->guest_pdptr2 = vmcs_read64(GUEST_PDPTR2);
- vmcs12->guest_pdptr3 = vmcs_read64(GUEST_PDPTR3);
+ if (nested_cpu_has_ept(vmcs12) && !is_long_mode(vcpu) &&
+ is_pae(vcpu) && is_paging(vcpu)) {
+ vmcs12->guest_pdptr0 = vmcs_read64(GUEST_PDPTR0);
+ vmcs12->guest_pdptr1 = vmcs_read64(GUEST_PDPTR1);
+ vmcs12->guest_pdptr2 = vmcs_read64(GUEST_PDPTR2);
+ vmcs12->guest_pdptr3 = vmcs_read64(GUEST_PDPTR3);
+ }
}

vmcs12->guest_linear_address = vmcs_readl(GUEST_LINEAR_ADDRESS);
--
2.21.0