linux-kernel - Re: [RFC PATCH 3/3] boot bzImages under paravirt

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Message-ID: <463B62B3.1030408@zytor.com>
Date:	Fri, 04 May 2007 09:43:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Zachary Amsden <zach@...are.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] boot bzImages under paravirt

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
>> In 32-bit mode?  Surely you're joking, Mr. Feynman!
>>   
> 
> Right, yes.
> 
>> What's worse, reloading segments here might be highly unsafe, if the
>> memory previously occupied by the GDT has been overwritten.  Keep in
>> mind the GDT is touched on a segment *load*, not on a segment *access*;
>> in areas such as booting that can be a huge difference.
>>   
> 
> Yep, suits me.  I'm happy for the code to assume that at least %cs and
> %ds are sane; I guess %ss too.  We could copy %ds into %[efg]s if we
> want to be sure (since I could imagine a bootloader leaving them in a
> less defined state).

No, we shouldn't.  %es should be assumed set up (this is 32-bit code,
after all!), and %fs and %gs should not be used.

> But if the gdt could be missing altogether, then, yes, we should not
> touch them at all.

Exactly.  Not relying on a set-up GDT is the safest option, IMNSHO.

	-hpa
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