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

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Date:	Mon, 5 Mar 2007 21:16:42 +0100
From:	Andi Kleen <ak@...e.de>
To:	Zachary Amsden <zach@...are.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	virtualization <virtualization@...ts.osdl.org>
Subject: Re: [patch] paravirt: VDSO page is essential

 We are not concerned so much 
> with supporting legacy user land deployments,

I am concerned about that. I won't merge any patches that break compatibility
by default.

> > What would probably work is to somehow decide at runtime if a hypervisor
> > is there or not and then set vdso default based on that. I guess that
> > detection would be hypervisor specific though and probably would
> > need paravirt ops extensions.
> >   
> 
> What we really need to do is to be able to detect an old user land and 
> drop VDSO support when that is found.  

Rusty implemented that, but it was widely considered too ugly
(and it was not 100% reliable e.g. with chroots) 

> But since we can't do that, the  
> next best thing is to allow the hypervisor to choose whatever workaround 
> it wants when it moves the fixmap and compat_vdso was enabled.  In our 
> case, the workaround we will want is a boot option to disable VDSO for 
> old user land,

The boot option is already there, but boot options for is not my
idea of user friendly binary compatibility.

> and a printk warning if you take #GPs and kill the init  
> proc, because for us, this is not an expected support scenario.  We 
> would much rather support the VDSO by default in paravirt kernels even 
> with COMPAT_VDSO turned on.

But you can't have it at the compatible fixed address, right?

-Andi
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