linux-kernel - Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

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Date:	Wed, 8 Jun 2011 12:35:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	pageexec@...email.hu, Andrew Lutomirski <luto@....edu>,
	x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Jesper Juhl <jj@...osbits.net>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>,
	richard -rw- weinberger <richard.weinberger@...il.com>,
	Mikael Pettersson <mikpe@...uu.se>,
	Brian Gerst <brgerst@...il.com>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	Valdis.Kletnieks@...edu
Subject: Re: [PATCH v5 8/9] x86-64: Emulate legacy vsyscalls

* Andi Kleen <andi@...stfloor.org> wrote:

> > eventually may even go away as time progresses and linux systems 
> > begin to fully rely on the vdso instead.
> 
> That assumes that everyone uses glibc and also updates their 
> userland. As pointed out many times that's a deeply flawed 
> assumption.

No, it does not assume it: if a particular usecase cares *so* little 
about updates that it won't ever update their userland and kernel 
then they have no problem: they'll have what they had before.

If they are willing to update the *kernel* then they will have to 
consider what every kernel update brings with itself: legacy 
facilities are de-emphasised all the time (while the ABI is still 
fully guaranteed) so user-space should not assume that newer kernels 
will offer the exact same performance tradeoffs as before.

Nor do i think have you cited any *real* example - you are just 
talking hypotheticals with very little specifics. ABI does not mean 
'executes the same instructions', guaranteeing that would be crazy.

Thanks,

	Ingo
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