[PATCH v2 13/13] KVM: nSVM: Only copy NP_ENABLE from VMCB01's misc_ctl

Yosry Ahmed
2025-11-10T22:30:05Z

The 'misc_ctl' field in VMCB02 is taken as-is from VMCB01. However, the
only bit that needs to copied is NP_ENABLE. This is a nop now because
other bits are for SEV guests, which do not support nested.
Nonetheless, this hardens against future bugs if/when other bits are
set for L1 but should not be set for L2.

Opportunistically add a comment explaining why NP_ENABLE is taken from
VMCB01 and not VMCB02.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
arch/x86/kvm/svm/nested.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 503cb7f5a4c5f..4e278c1f9e6b3 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -837,8 +837,16 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
V_NMI_BLOCKING_MASK);
}

- /* Copied from vmcb01. msrpm_base can be overwritten later. */
- vmcb02->control.misc_ctl = vmcb01->control.misc_ctl;
+ /*
+ * Copied from vmcb01. msrpm_base can be overwritten later.
+ *
+ * NP_ENABLE in vmcb12 is only used for consistency checks. If L1
+ * enables NPTs, KVM shadows L1's NPTs and uses those to run L2. If L1
+ * disables NPT, KVM runs L2 with the same NPTs used to run L1. For the
+ * latter, L1 runs L2 with shadow page tables that translate L2 GVAs to
+ * L1 GPAs, so the same NPTs can be used for L1 and L2.
+ */
+ vmcb02->control.misc_ctl = vmcb01->control.misc_ctl & SVM_MISC_CTL_NP_ENABLE;
vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;

--
2.51.2.1041.gc1ab5b90ca-goog