KVM: VMX: Cache vmcs segment fields

Since the emulator now checks segment limits and access rights, it
generates a lot more accesses to the vmcs segment fields.  Undo some
of the performance hit by cacheing those fields in a read-only cache
(the entire cache is invalidated on any write, or on guest exit).

Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -136,6 +136,7 @@ enum kvm_reg_ex {
 	VCPU_EXREG_CR3,
 	VCPU_EXREG_RFLAGS,
 	VCPU_EXREG_CPL,
+	VCPU_EXREG_SEGMENTS,
 };
 
 enum {