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

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Message-ID: <463B4E12.50703@goop.org>
Date:	Fri, 04 May 2007 08:15:30 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] boot bzImages under paravirt

Eric W. Biederman wrote:
> Ok.  Although we can hoist the bss zeroing, if everything needs it.
>   

It will if we're booting out of bzImage; the bss won't be clear in that
case.

> Hmm.  I'm wondering about the segment reload and how much of a problem
> that is.  My memory says that segment reloads are not actually a
> privileged operation, so we may be able to support this even in
> paravirt mode.  How hard would that be to support?  The segment
> we reload is a fixed part of our boot protocol.
>   

The problem is not the reloads themselves, but what you're reloading
them with.  If we come up under Xen, then it will provide a default GDT
and pre-load the segments with flat 4G(-ish) selectors - but the
selectors won't be the normal Linux ones.

So if we reload using a constant selector, then that will break under
Xen. But if we do a:

    mov %cs, %eax
    mov %eax, %ds
    // etc

sequence then it should be fine.  This will work even for loading %ss,
since the %cs CPL will equal the RPL needed for %ss.

    J
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