Re: Threads and the LDT (Intel-specific)?

[Andi Kleen <ak@suse.de>]

Scott Long <smlong@teleport.com> writes:

> I can also use the LDT to point to thread-specific segments. IMHO this 
> is much better than the stack trick used by linuxthreads. The problem 

Modern LinuxThreads (glibc 2.2) also uses modify_ldt for thread local data
(much to the pain of the IA64 and x86-64 ports who have to emulate it..) 

> is, I don't fully understand how to use modify_ldt(). Is anyone 
> knowledgeable about this?

modify_ldt() works like a memcpy to/from the ldt. See the man page. On the
layout of the LDT see the intel IA32 architecture manual from http://developer.intel.com
 
> If anyone has any other suggestions, please let me know. If you are 
> confused as to why I would ever want to do this in the first place, I'd 
> be willing to go over it off the list.

I can imagine, but it'll cost you. Most modern CPUs have heavy penalties for
non-zero-based or limited segments (a few cycles for every memory access) and LDT
switching also makes the context switch slower.

An portable alternative is to use multiple processes with a shared memory area.

-Andi