KVM: nVMX: Emulate guest TLB flush on nested VM-Enter with new vpid12

Fully emulate a guest TLB flush on nested VM-Enter which changes vpid12,
i.e. L2's VPID, instead of simply doing INVVPID to flush real hardware's
TLB entries for vpid02.  From L1's perspective, changing L2's VPID is
effectively a TLB flush unless "hardware" has previously cached entries
for the new vpid12.  Because KVM tracks only a single vpid12, KVM doesn't
know if the new vpid12 has been used in the past and so must treat it as
a brand new, never been used VPID, i.e. must assume that the new vpid12
represents a TLB flush from L1's perspective.

For example, if L1 and L2 share a CR3, the first VM-Enter to L2 (with a
VPID) is effectively a TLB flush as hardware/KVM has never seen vpid12
and thus can't have cached entries in the TLB for vpid12.
Reported-by: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Fixes: 5c614b35 ("KVM: nVMX: nested VPID emulation")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125014944.536398-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -1162,29 +1162,26 @@ static void nested_vmx_transition_tlb_flush(struct kvm_vcpu *vcpu,
 	WARN_ON(!enable_vpid);
 
 	/*
-	 * If VPID is enabled and used by vmc12, but L2 does not have a unique
-	 * TLB tag (ASID), i.e. EPT is disabled and KVM was unable to allocate
-	 * a VPID for L2, flush the current context as the effective ASID is
-	 * common to both L1 and L2.
-	 *
-	 * Defer the flush so that it runs after vmcs02.EPTP has been set by
-	 * KVM_REQ_LOAD_MMU_PGD (if nested EPT is enabled) and to avoid
-	 * redundant flushes further down the nested pipeline.
-	 *
-	 * If a TLB flush isn't required due to any of the above, and vpid12 is
-	 * changing then the new "virtual" VPID (vpid12) will reuse the same
-	 * "real" VPID (vpid02), and so needs to be flushed.  There's no direct
-	 * mapping between vpid02 and vpid12, vpid02 is per-vCPU and reused for
-	 * all nested vCPUs.  Remember, a flush on VM-Enter does not invalidate
-	 * guest-physical mappings, so there is no need to sync the nEPT MMU.
-	 */
-	if (!nested_has_guest_tlb_tag(vcpu)) {
-		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
-	} else if (is_vmenter &&
-		   vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
+	 * VPID is enabled and in use by vmcs12.  If vpid12 is changing, then
+	 * emulate a guest TLB flush as KVM does not track vpid12 history nor
+	 * is the VPID incorporated into the MMU context.  I.e. KVM must assume
+	 * that the new vpid12 has never been used and thus represents a new
+	 * guest ASID that cannot have entries in the TLB.
+	 */
+	if (is_vmenter && vmcs12->virtual_processor_id != vmx->nested.last_vpid) {
 		vmx->nested.last_vpid = vmcs12->virtual_processor_id;
-		vpid_sync_context(nested_get_vpid02(vcpu));
+		kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
+		return;
 	}
+
+	/*
+	 * If VPID is enabled, used by vmc12, and vpid12 is not changing but
+	 * does not have a unique TLB tag (ASID), i.e. EPT is disabled and
+	 * KVM was unable to allocate a VPID for L2, flush the current context
+	 * as the effective ASID is common to both L1 and L2.
+	 */
+	if (!nested_has_guest_tlb_tag(vcpu))
+		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 }
 
 static bool is_bitwise_subset(u64 superset, u64 subset, u64 mask)