linux-kernel - Re: [patch] paravirt: VDSO page is essential

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Date:	Tue, 6 Mar 2007 09:34:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Roland McGrath <roland@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jan Beulich <jbeulich@...ell.com>
Subject: Re: [patch] paravirt: VDSO page is essential

* Roland McGrath <roland@...hat.com> wrote:

> > Jan Beulich just posted a patch to do just this - relocate the 
> > vdso's ELF header.  If that's all that's really required to keep 
> > COMPAT_VDSO viable under PARAVIRT, then it seems like the way to go.
> 
> I found 
> http://marc.theaimsgroup.com/?l=xen-devel&m=117309332600075&w=2 and 
> that must be the one you meant.  The ELF-grokking form of that is 
> exactly what I had in mind.  The "find relocs with cmp" scheme is 
> pretty silly, but also works fine.  It trades tweaky ELF knowledge 
> with tweaky fragile build methods, but it's all about the same to me.

this looks good to me too in principle, the #else branch. But the actual 
implementation will have to be redone quite a bit i fear. Some details: 
relocate_vdso() needs some major coding style cleanups. This bit:

-# define VDSO_PRELINK          VDSO_HIGH_BASE
+# ifndef CONFIG_XEN
+#  define VDSO_PRELINK         VDSO_HIGH_BASE
+# else
+#  define VDSO_PRELINK         (0UL - FIX_VDSO * PAGE_SIZE)
+# endif

should be Kconfig driven, not #ifdef driven, due to cleanliness and also 
because lguest wants to have the same thing. Plus:

+#if defined(CONFIG_XEN) && defined(CONFIG_COMPAT_VDSO)

i'd just make this depend on CONFIG_COMPAT_VDSO, always. Same here:

+#if defined(CONFIG_XEN) && defined(CONFIG_COMPAT_VDSO)
+static void __init relocate_vdso

just make this driven in the normal CONFIG_COMPAT_VDSO case too - even 
though we 'prelink' the VDSO to the usual address - we better run 
through the same code all the time and reduce the number of variants as 
much as possible.

furthermore, there should be a paravirt_ops method to chose the 
relocation address, unless i'm missing something. On the native kernel 
that address will default to 0xffffe000. (if CONFIG_COMPAT_VDSO is 
selected)

this way there will only be two main variants to worry about: compat and 
modern (which is the current status quo anyway), instead of 4-5 
variants.

	Ingo
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