[gentoo-server] USE=nptlonly and -mno-tls-direct-seg-refs -- when to use?

[gentoo-server] USE=nptlonly and -mno-tls-direct-seg-refs -- when to use?

[Pandu Poluan]

Since I'm trying to optimize Gentoo on XenServer (as DomU, *not*
Dom0), I think this question is best addressed to gentoo-server list.

When I was emerging glibc, I got this warning:

 * You are using Xen but don't have -mno-tls-direct-seg-refs in your CFLAGS.
 * This will result in a 50% performance penalty when running with a 32bit
 * hypervisor, which is probably not what you want.

A google-fu brought me to this page:

http://cs.felk.cvut.cz/~vaclam1/linux/ucebna/k310/data/doc/howto-xen-and-gentoo-en.html#Before_we_begin

There, it mentions 2 things to be done:

  * -mno-tls-direct-seg-refs in CFLAGS
  * USE=nptlonly for sys-libs/glibc -- which I assume has been
superseded with the global USE=nptl

Now my questions:

1. When do we use either (or both)?

1a. I'm compiling Gentoo x86 on XenServer running on an x86-64 (Intel)
system, do I need them?

1b. I'm compiling Gentoo amd64 on XenServer running on an x86-64
(Intel) system, do I need them?

1c. Like (1a), but after completion, I'll migrate the VM onto a
XenServer running on a Pentium4 system (32-bit), do I need them?

(In other words, it's built on top of an x86-64 physical platform, but
will be running on top of an x86 physical platform)

TIA for your assistance.

Rgds,
-- 
Pandu E Poluan
~ IT Optimizer ~
Visit my Blog: http://pepoluan.posterous.com