Boot hang on Xen after r318347/(310418)

[Adam McDougall]

Hello,

Recently I made a new build of 11-STABLE but encountered a boot hang
at this state:
http://www.egr.msu.edu/~mcdouga9/pics/r318347-smp-hang.png

It is easy to reproduce, I can just boot from any 11 or 12 ISO that 
contains the commit.

I compiled various svn revisions to confirm that r318347 caused the 
issue and r318346 is fine. With r318347 or later including the latest 
11-STABLE, the system will only boot with one virtual CPU in XenServer. 
Any more cpus and it hangs. I also tried a 12 kernel from head this 
afternoon and I have the same hang. I had this issue on XenServer 7 
(Xen 4.7) and XenServer 6.5 (Xen 4.4). I did most of my testing on 7. I 
also did much of my testing with a GENERIC kernel to try to rule out 
kernel configuration mistakes. When it hangs, the performance 
monitoring in Xen tells me at least one CPU is pegged. r318674 boots 
fine on physical hardware without Xen involved.

Looking at r318347 which mentions EARLY_AP_STARTUP and later seeing 
r318763 which enables EARLY_AP_STARTUP in GENERIC, I tried adding it to 
my kernel but it turned the hang into a panic but with any number of 
CPUs: 
http://www.egr.msu.edu/~mcdouga9/pics/r318347-early-ap-startup-panic.png

I think I verified that this happens with EARLY_AP_STARTUP before
r318347 too so I'll assume it is a different problem.

I may need to do some experimentation to figure out how to get the 
console to pass through hotkeys to drop into a kernel debugger. I could 
also try modifying the kernel config if I can make it print information 
about the hang.

Is there anything else I can provide that might help? Would you prefer 
this be entered in a bugzilla report? Thanks.