KVM: x86: SVM: don't save SVM state to SMRAM when VM is not long mode capable

When the guest CPUID doesn't have support for long mode, 32 bit SMRAM
layout is used and it has no support for preserving EFER and/or SVM
state.

Note that this isn't relevant to running 32 bit guests on VM which is
long mode capable - such VM can still run 32 bit guests in compatibility
mode.
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20221025124741.228045-23-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/svm.c

@@ -4410,6 +4410,14 @@ static int svm_enter_smm(struct kvm_vcpu *vcpu, union kvm_smram *smram)
 	if (!is_guest_mode(vcpu))
 		return 0;
 
+	/*
+	 * 32-bit SMRAM format doesn't preserve EFER and SVM state.  Userspace is
+	 * responsible for ensuring nested SVM and SMIs are mutually exclusive.
+	 */
+
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_LM))
+		return 1;
+
 	smram->smram64.svm_guest_flag = 1;
 	smram->smram64.svm_guest_vmcb_gpa = svm->nested.vmcb12_gpa;