linux-kernel - Re: Assignment of GDT entries

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Date:	Wed, 13 Sep 2006 15:22:25 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...l.org>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Zachary Amsden <zach@...are.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael A Fetterman <Michael.Fetterman@...cam.ac.uk>
Subject: Re: Assignment of GDT entries

Linus Torvalds wrote:
> So I'd not be surprised if movign the TLS segments around would break 
> something. 
>   

I don't think so.  32-bit code running on x86-64 has different TLS 
selectors, and everything seems to work there...

> That said, numbers talk, bullshit walks. If the above just works a lot 
> better for all modern CPU's that all have 64-byte cachelines (because now 
> _everything_ is in that bigger cacheline), and if you can show that with 
> numbers, and nothing breaks in practice, then hey..
>   

My goal would be to do a minimal change which packs all the useful stuff 
together in a 64-byte line.  Ideally it would just use two 32-byte 
lines, but I don't think that's as important.

Caching effects are pretty hard to measure anyway, and with something as 
deeply x86-microarchitectural as this, I could imagine lots of other CPU 
cleverness which could obscure any simple measurement.  But packing 
things into a line certainly can't hurt.

I'll put something together, and see how it goes...

    J
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