kernel: x86 x2apic + deferred struct-page init

cloud-hypervisor describes vCPUs as x2apic MADT entries; without
CONFIG_X86_X2APIC the kernel ignored them and every x86_64 guest booted
with a single CPU regardless of --cpus. DEFERRED_STRUCT_PAGE_INIT moves
the largest silent stretch of early boot onto all CPUs (x86 guest boot
202 -> 130 ms); measured as a wash on arm64 (VZ and KVM), so only x86
takes it.

Co-authored-by: Hanzo Dev <[email protected]>

kernel/vm_defconfig

@@ -8,6 +8,9 @@ CONFIG_ARM64=y
 CONFIG_ARCH_VIRT=y
 CONFIG_SMP=y
 CONFIG_NR_CPUS=16
+# DEFERRED_STRUCT_PAGE_INIT measured as a wash here (VZ and KVM, 2 GB
+# guests): the deferred pass costs what the serial pass saved. x86 keeps
+# it because cloud-hypervisor guests measurably win.
 
 # No loadable modules
 # CONFIG_MODULES is not set

kernel/vm_defconfig_x86

@@ -7,6 +7,12 @@
 CONFIG_64BIT=y
 CONFIG_SMP=y
 CONFIG_NR_CPUS=16
+# cloud-hypervisor describes vCPUs as x2apic MADT entries; without this the
+# kernel ignores them and boots one CPU no matter how many were given.
+CONFIG_X86_X2APIC=y
+# Initialize struct pages on all CPUs after smp_init instead of serially on
+# the boot CPU — trims the single largest silent stretch of early boot.
+CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
 CONFIG_HYPERVISOR_GUEST=y
 CONFIG_PARAVIRT=y
 CONFIG_KVM_GUEST=y