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Date: Sun, 12 Nov 2000 20:33:53 +0100
From: Andi Kleen <ak@suse.de>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrea Arcangeli <andrea@suse.de>,
        Tigran Aivazian <tigran@aivazian.fsnet.co.uk>,
        Tigran Aivazian <tigran@veritas.com>,
        "H. Peter Anvin" <hpa@transmeta.com>, Max Inux <maxinux@bigfoot.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org
Subject: Re: bzImage ~ 900K with i386 test11-pre2
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In-Reply-To: <m1bsvlauic.fsf@frodo.biederman.org>; from ebiederm@xmission.com on Sun, Nov 12, 2000 at 11:57:15AM -0700
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[This is quite a bizarre discussion, but I'll answer anyways. I am not exactly
sure what your point is]

On Sun, Nov 12, 2000 at 11:57:15AM -0700, Eric W. Biederman wrote:
> 
> > > I can tell you don't have real hardware.  The non obviousness
> 
> I need to retract this a bit.  You are still building a compressed image,
> and the code in the boot/compressed/head.S remains unchanged and loads
> segment registers, so it works by luck.  If you didn't build a
> compressed image you would be in trouble.

boot/compressed/head.S does run in 32bit legacy mode, where you of course
need segment registers. After you got into long mode segments are only
needed to jump between 32/64bit code segments and and for a the data segment
of the 32bit emulation (+ the iretd bug currently which I hope will be fixed
in final hardware) 

Also note that boot/compressed/* currently does not even link, because the 
x86-64 toolchain cannot generate relocated 32bit code ATM (the linker chokes
on the 32bit relocations) The tests we did so far used a precompiled 
relocated binary compressed/{head,misc}.o from a IA32 build.

> > 	In 64-bit mode, the contents of the ES, DS, and SS segment registers
> > 	are ignored. All fields (base, limit, and attribute) in the
> > 	corresponding segment descriptor registers (hidden part) are also
> > 	ignored.
> 
> Hmm.  I'll have to look and see if FS & GS are also ignored.

They are not, you to fully use them you need privileged MSRs. 
Their limit is ignored.

> > Sure, go ahead if you weren't missing that basic part of the long mode specs.
> > Thanks.
> 
> Nope.  Though I suspect we should do the switch to 64bit mode in
> setup.S and not have these issues pollute head.S at all.

I see no advantage in doing it there instead of in head.S



-Andi
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