sysenter/syscall support for 32-on-64 guests?

[Jeremy Fitzhardinge <jeremy@goop.org>]

I just spent quite a while working out why pvops-Xen kernels weren't 
booting for me in a 64-bit hypervisor.  It turned out to be because 
32-on-64 supports SEP even though 32-on-32 doesn't, so the guest kernel 
was trying to use it without having set up the Xen side, resulting in 
syscalls jumping to 0:0, generating a mysterious GP.  Once I worked that 
out, the fix was simple, of course.

Anyway, I wonder if you had a specific reason for allowing this, other 
than "because we can"?  It seems to me that we should try to minimize 
the number of differences between 32-on-32 and 32-on-64 as much as possible.

    J