[PATCH v5 26/26] KVM: nSVM: Only copy SVM_MISC_ENABLE_NP from VMCB01's misc_ctl

[Yosry Ahmed <yosry.ahmed@linux.dev>]

The 'misc_ctl' field in VMCB02 is taken as-is from VMCB01. However, the
only bit that needs to copied is SVM_MISC_ENABLE_NP, as all other known
bits in misc_ctl are related to SEV guests, and KVM doesn't support
nested virtualization for SEV guests.

Only copy SVM_MISC_ENABLE_NP to harden against future bugs if/when other
bits are set for L1 but should not be set for L2.

Opportunistically add a comment explaining why SVM_MISC_ENABLE_NP 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 c87738962970..d80b1bde6630 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -852,8 +852,16 @@ 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.
+	 *
+	 * SVM_MISC_ENABLE_NP 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_ENABLE_NP;
 	vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
 	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
 	vmcb_mark_dirty(vmcb02, VMCB_PERM_MAP);
-- 
2.53.0.rc2.204.g2597b5adb4-goog