Changing semantics of ioperm() on Xen x86-64?

[Anthony Liguori <aliguori@us.ibm.com>]

As part of the Xen x86-64 Linux port, we've changed the ioperm() syscall 
to always modify the IOPL instead of actually modifying the IO bitmap in 
the TSS like we do on x86-32.  Is there a particular reason for doing this?

I'm completely guessing here that this may allow us to avoid changing 
the TR when changing from user/kernel mode but that doesn't seem like 
that huge of a gain.

I don't expect that there are many apps that would rely on using ioperm 
to restrict access to only certain ranges of ports so I don't think this 
is a security problem but it still is a little discomforting.

Comments?

Regards,

Anthony Liguori