From owner-freebsd-hackers  Tue Dec  7 21:53:57 1999
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Date: Wed, 8 Dec 1999 00:53:51 -0500 (EST)
From: Luoqi Chen <luoqi@watermarkgroup.com>
Message-Id: <199912080553.AAA03932@lor.watermarkgroup.com>
To: hackers@FreeBSD.ORG, vsilyaev@mindspring.com
Subject: Re:  FreeBSD-i386 and GS selector register
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> Hi
> 
> I have the next question.
> The FreeBSD on i386 don't use GS register, even kernel DDB don't show
> it. And at the time when kernel loaded and operational GS didn't 
> initialized yet and have some garbage value (something like 0x1f, i.e.
> pointed to the LDT). But because no one don't touch that all working fine.
> 
0x1f is not garbage, it's (FreeBSD) standard user data segment.

> Question.
> The some driver had code like this:
> 	push	%gs <--- OK 0x1f  saved on the stack
> 	...  
> 	call    do_big_deal
> 	...
> 	pop	%gs <--- Restore 0x1f and have a fault, probably Double Fault
> 
It could only be that the driver code changed the LDT descriptor and didn't
restore it upon return.

> I can't change that code. When I found this problem, I did simple hack,
> before call that code I'm clear GS. But I want to know may be exist
> a better solution?
> 
> -- 
> Vladimir Silyaev
> 

-lq


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