Breaking loop by checking iPXE special feature?

[foresty]

Sorry for posting my question here!
Because I can't find where is the button to post a new discussion.
(Maybe I have no right to post question as a new user).

Seriously, I have to register my new same accout(foresty, with the same PASSWORD for 3rd time), as I can't keep my account after registering about 5 hours.

##### My question is #####
I have met an issue when use iPXE booting VM on Ctrix XenServer 6.5.
The VM has an old existing iPXE verision embeded. I got failed chainload old iPXE ROM with
"user-class iPXE" option.
if exists user-class and option user-class = "iPXE" {
filename "http://pxe.mydomain.org/ipxe/boot.ipxe";
} else { ...

Follow Robin's link "https://gist.github.com/robinsmidsrod/4008017"
I also tried to use "iPXE feature detetion" on my DHCP by replacing user-class option (it works before changing), but it didn't break iPXE loop:
if exists ipxe.https {
filename "http://pxe.mydomain.org/ipxe/boot.ipxe";
} else { ...

So anybody could help me? Thanks very much.