x86/tss: Fix and move VMX BUILD_BUG_ON()

The BUILD_BUG_ON(IO_BITMAP_OFFSET - 1 == 0x67) in the VMX code is bogus in
two aspects:

1) This wants to be in generic x86 code simply to catch issues even when
   VMX is disabled in Kconfig.

2) The IO_BITMAP_OFFSET is not the right thing to check because it makes
   asssumptions about the layout of tss_struct. Nothing requires that the
   I/O bitmap is placed right after x86_tss, which is the hardware mandated
   tss structure. It pointlessly makes restrictions on the struct
   tss_struct layout.

The proper thing to check is:

    - Offset of x86_tss in tss_struct is 0
    - Size of x86_tss == 0x68

Move it to the other build time TSS checks and make it do the right thing.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Andy Lutomirski <luto@kernel.org>

arch/x86/mm/cpu_entry_area.c

@@ -161,6 +161,14 @@ static void __init setup_cpu_entry_area(unsigned int cpu)
 	BUILD_BUG_ON((offsetof(struct tss_struct, x86_tss) ^
 		      offsetofend(struct tss_struct, x86_tss)) & PAGE_MASK);
 	BUILD_BUG_ON(sizeof(struct tss_struct) % PAGE_SIZE != 0);
+	/*
+	 * VMX changes the host TR limit to 0x67 after a VM exit. This is
+	 * okay, since 0x67 covers the size of struct x86_hw_tss. Make sure
+	 * that this is correct.
+	 */
+	BUILD_BUG_ON(offsetof(struct tss_struct, x86_tss) != 0);
+	BUILD_BUG_ON(sizeof(struct x86_hw_tss) != 0x68);
+
 	cea_map_percpu_pages(&cea->tss, &per_cpu(cpu_tss_rw, cpu),
 			     sizeof(struct tss_struct) / PAGE_SIZE, tss_prot);