kvm: vmx: Document the need for MSR_STAR in i386 builds

Add a comment explaining why MSR_STAR must be included in
vmx_msr_index[] even for i386 builds.

The elided comment has not been relevant since move_msr_up() was
introduced in commit a75beee6 ("KVM: VMX: Avoid saving and
restoring msrs on lightweight vmexit").
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -373,8 +373,11 @@ static const struct kvm_vmx_segment_field {
 u64 host_efer;
 
 /*
- * Keep MSR_STAR at the end, as setup_msrs() will try to optimize it
- * away by decrementing the array size.
+ * Though SYSCALL is only supported in 64-bit mode on Intel CPUs, kvm
+ * will emulate SYSCALL in legacy mode if the vendor string in guest
+ * CPUID.0:{EBX,ECX,EDX} is "AuthenticAMD" or "AMDisbetter!" To
+ * support this emulation, IA32_STAR must always be included in
+ * vmx_msr_index[], even in i386 builds.
  */
 const u32 vmx_msr_index[] = {
 #ifdef CONFIG_X86_64