linux-kernel - Re: [PATCH v2] x86: Remove compat vdso support

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Date:	Tue, 11 Mar 2014 09:42:42 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefani Seibold <stefani@...bold.net>,
	the arch/x86 maintainers <x86@...nel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>
Subject: Re: [PATCH v2] x86: Remove compat vdso support

On 03/11/2014 09:30 AM, Linus Torvalds wrote:
> 
> No, the trivial solution is to stop adding crap to it.
> 
> And no, "just reserve a little more space for it" is neither trivial
> nor a good idea. The fixed VDSO address is very much at the top of the
> address space, so you can't allocate more space for it unless you do
> one of
> 
>  (a) make it non-contiguous
>  (b) get rid of the hole that is the very last page
>  (c) mess with the vsyscall pages and make it contiguous "backwards"
> 
> all of which sound like *horrible* ideas. Certainly not "trivial solution".
> 
> No, the trivial solution is to not mess with that legacy page at all.
> 
> Why is *that* trivial solution not on the table? Why the heck are
> people hell-bent on changing this stupid legacy page around?
> 
> I find this whole thread very annoying. We shouldn't care about
> x86-32, and certainly not from a performance angle - we should
> consider it a "it's done, don't touch it" issue.
> 

Andy actually did the research, and found that even the legacy VDSO
doesn't have to live at any one particular address, it just has to live
at the address it is linked at.  So we can move it just fine, but we
have to change the link address to match.

That gives us a lot more maneuvering room than saying it has to be at
one specific address.

	-hpa


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