x86/kvm: Enable 15-bit extension when KVM_FEATURE_MSI_EXT_DEST_ID detected

This allows the host to indicate that MSI emulation supports 15-bit
destination IDs, allowing up to 32768 CPUs without interrupt remapping.

cf. https://patchwork.kernel.org/patch/11816693/ for qemu
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20201024213535.443185-36-dwmw2@infradead.org

arch/x86/kernel/kvm.c

@@ -740,6 +740,11 @@ static void __init kvm_apic_init(void)
 #endif
 }
 
+static bool __init kvm_msi_ext_dest_id(void)
+{
+	return kvm_para_has_feature(KVM_FEATURE_MSI_EXT_DEST_ID);
+}
+
 static void __init kvm_init_platform(void)
 {
 	kvmclock_init();
@@ -769,6 +774,7 @@ const __initconst struct hypervisor_x86 x86_hyper_kvm = {
 	.type				= X86_HYPER_KVM,
 	.init.guest_late_init		= kvm_guest_init,
 	.init.x2apic_available		= kvm_para_available,
+	.init.msi_ext_dest_id		= kvm_msi_ext_dest_id,
 	.init.init_platform		= kvm_init_platform,
 #if defined(CONFIG_AMD_MEM_ENCRYPT)
 	.runtime.sev_es_hcall_prepare	= kvm_sev_es_hcall_prepare,