x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE

vmx_save_host_state() is only called from kvm_arch_vcpu_ioctl_run() so
the context is pretty well defined and as we're past 'swapgs' MSR_GS_BASE
should contain kernel's GS base which we point to irq_stack_union.

Add new kernelmode_gs_base() API, irq_stack_union needs to be exported
as KVM can be build as module.

Acked-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kernel/cpu/common.c

@@ -487,7 +487,7 @@ void load_percpu_segment(int cpu)
 	loadsegment(fs, __KERNEL_PERCPU);
 #else
 	__loadsegment_simple(gs, 0);
-	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
+	wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
 #endif
 	load_stack_canary_segment();
 }
@@ -1398,6 +1398,7 @@ __setup("clearcpuid=", setup_clearcpuid);
 #ifdef CONFIG_X86_64
 DEFINE_PER_CPU_FIRST(union irq_stack_union,
 		     irq_stack_union) __aligned(PAGE_SIZE) __visible;
+EXPORT_PER_CPU_SYMBOL_GPL(irq_stack_union);
 
 /*
  * The following percpu variables are hot.  Align current_task to