[PATCH 2/3] KVM: nSVM: Do not track EFER.SVME toggling in guest mode

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

KVM tracks when EFER.SVME is set and cleared to initialize and tear down
nested state. However, it doesn't differentiate if EFER.SVME is getting
toggled in L1 or L2+. Toggling EFER.SVME in L2+ is inconsequential from
KVM's perspective, as the vCPU is still obviously using nested.

This causes a problem if L2 sets then clears EFER.SVME without L1
interception, as KVM exits guest mode and tears down nested state while
L2 is running, executing L1 without injecting a proper #VMEXIT.

Technically, it's not a bug as the APM states that an L1 hypervisor
should intercept EFER writes:

	The effect of turning off EFER.SVME while a guest is running is
	undefined; therefore, the VMM should always prevent guests from
	writing EFER.

However, it would be nice if KVM handled it more gracefully.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 arch/x86/kvm/svm/svm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4575a6a7d6c4e..eaf0f8053fbfb 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -208,6 +208,13 @@ static int svm_set_efer_svme(struct kvm_vcpu *vcpu, u64 old_efer, u64 new_efer)
 	if ((old_efer & EFER_SVME) == (new_efer & EFER_SVME))
 		return 0;
 
+	/*
+	 * An L2 guest setting or clearing EFER_SVME does not change whether or
+	 * not the vCPU can use nested from KVM's perspective.
+	 */
+	if (is_guest_mode(vcpu))
+		return 0;
+
 	if (new_efer & EFER_SVME) {
 		r = svm_allocate_nested(svm);
 		if (r)
-- 
2.53.0.rc1.225.gd81095ad13-goog