Issue with PV superpage handling

[Dave McCracken <dcm@mccr.org>]

Awhile back I added the domain config flag "superpages" to support Linux 
hugepages in PV domains.  When the flag is set, the PV domain is populated 
entirely with superpages.  If not enough superpage-sized chunks can be found, 
the domain creation fails.

At some time after my patch was accepted, the code I added to domain restore 
was removed because I broke page allocation batching.  I put it on my TODO 
list to reimplement it, then it got lost, for which I apologize.

Now I have gotten back to reimplementing PV superpage support in restore, I 
find that recently other code was added to restore that, while triggered by 
the superpage flag, only allocates superpages opportunistically and falls back 
to small pages if it fails.  This breaks the original semantics of the flag 
and could cause any OS that depends on the semantics to fail catastrophically.

I have a patch that implements the original semantics of the superpage flag 
while preserving the batch allocation behavior.  I can remove the competing 
code and submit mine, but I have a question.  What value is there in 
implementing opportunistic allocation of superpages for a PV (or an HVM) 
domain in restore?  It clearly can't be based on the superpages flag.  
Opportunistic superpage allocation is already the default behavior for HVM 
domain creation.  Should it also be a default on HVM restore?  What about for 
PV domains?  Is there any real benefit?

Thanks,
Dave McCracken
Oracle Corp.