KVM: VMX: Avoid saving and restoring msr_efer on lightweight vmexit

MSR_EFER.LME/LMA bits are automatically save/restored by VMX
hardware, KVM only needs to save NX/SCE bits at time of heavy
weight VM Exit. But clearing NX bits in host envirnment may
cause system hang if the host page table is using EXB bits,
thus we leave NX bits as it is. If Host NX=1 and guest NX=0, we
can do guest page table EXB bits check before inserting a shadow
pte (though no guest is expecting to see this kind of gp fault).
If host NX=0, we present guest no Execute-Disable feature to guest,
thus no host NX=0, guest NX=1 combination.

This patch reduces raw vmexit time by ~27%.

Me: fix compile warnings on i386.

Signed-off-by: Yaozu (Eddie) Dong <eddie.dong@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm.h

@@ -255,6 +255,7 @@ struct kvm_stat {
 	u32 request_irq_exits;
 	u32 irq_exits;
 	u32 light_exits;
+	u32 efer_reload;
 };
 
 struct kvm_vcpu {
@@ -289,6 +290,7 @@ struct kvm_vcpu {
 	u64 ia32_misc_enable_msr;
 	int nmsrs;
 	int save_nmsrs;
+	int msr_offset_efer;
 #ifdef CONFIG_X86_64
 	int msr_offset_kernel_gs_base;
 #endif