RFC: Getting rid of LTR in VMX

[Andy Lutomirski <luto@amacapital.net>]

There's no code here because the patch is trivial, but I want to run
the idea by you all first to see if there are any issues.

VMX is silly and forces the TSS limit to the minimum on VM exits.  KVM
wastes lots of cycles bumping it back up to accomodate the io bitmap.

I propose that we rework this.  Add a percpu variable that indicates
whether the TSS limit needs to be refreshed.  On task switch, if the
new task has TIF_IO_BITMAP set, then check that flag and, if set,
refresh TR and clear the flag.  On VMX exit, set the flag.

The TSS limit is (phew!) invisible to userspace, so we don't have ABI
issues to worry about here.  We also shouldn't have security issues
because a too-low TSS limit just results in unprivileged IO operations
generating #GP, which is exactly what we want.

What do you all think?  I expect a speedup of a couple hundred cycles
on each VM exit.