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

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <andi@firstfloor.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