target/i386: Allow elision of kvm_enable_x2apic()

Call kvm_enabled() before kvm_enable_x2apic() to let the compiler elide
its call.  Cleanup the code by simplifying "!xen_enabled() &&
kvm_enabled()" to just "kvm_enabled()".

Suggested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230904124325.79040-8-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/intel_iommu.c

@@ -4053,7 +4053,7 @@ static bool vtd_decide_config(IntelIOMMUState *s, Error **errp)
             error_setg(errp, "eim=on requires accel=kvm,kernel-irqchip=split");
             return false;
         }
-        if (!kvm_enable_x2apic()) {
+        if (kvm_enabled() && !kvm_enable_x2apic()) {
             error_setg(errp, "eim=on requires support on the KVM side"
                              "(X2APIC_API, first shipped in v4.7)");
             return false;

hw/i386/x86.c

@@ -129,13 +129,10 @@ void x86_cpus_init(X86MachineState *x86ms, int default_cpu_version)
                                                       ms->smp.max_cpus - 1) + 1;
 
     /*
-     * Can we support APIC ID 255 or higher?
-     *
-     * Under Xen: yes.
-     * With userspace emulated lapic: no
-     * With KVM's in-kernel lapic: only if X2APIC API is enabled.
+     * Can we support APIC ID 255 or higher?  With KVM, that requires
+     * both in-kernel lapic and X2APIC userspace API.
      */
-    if (x86ms->apic_id_limit > 255 && !xen_enabled() &&
+    if (x86ms->apic_id_limit > 255 && kvm_enabled() &&
         (!kvm_irqchip_in_kernel() || !kvm_enable_x2apic())) {
         error_report("current -smp configuration requires kernel "
                      "irqchip and X2APIC API support.");

target/i386/kvm/kvm-stub.c

@@ -12,13 +12,6 @@
 #include "qemu/osdep.h"
 #include "kvm_i386.h"
 
-#ifndef __OPTIMIZE__
-bool kvm_enable_x2apic(void)
-{
-    return false;
-}
-#endif
-
 bool kvm_hv_vpindex_settable(void)
 {
     return false;