Re: [Xen-users] 64bit processors and TLS warning message

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > > I am running an AMD64 Debian system and I noticed that the warning
> > > message about /lib/tls like on my other 32bit systems does not appear
> > > on this one. Does that mean that AM64 Linux archs aren't TLS enabled ?
> >
> > It's just not an issue like it is on 32-bit - the TLS implementation
> > doesn't conflict with the way Xen enforces protection.
>
> If by "different implementation" you mean that "different TLS
> implementations in Xen are used for different hardware architecures",
> that makes me wonder:

I rather suspected they might have implemented glibc's TLS support in a 
slightly more conventional way on x86_64 (i.e. not using the cunning segment 
trick).  This is apparently not the case, so the only difference is in the 
way Xen protects itself from the guest.  It's still the case that glibc's TLS 
on x86-64 should be perfectly safe and performant under Xen.

> If I do *not* use -mno-tls-direct-seg-refs or gcc patches on my x86-64
> system, will I actually enjoy a performance benefit from the GCC
> compiler's TLS optimizations, at least for 32bit userland apps?

In theory, yes, this might speed things up since it'll improve the efficiency 
of access to TLS data.  In practice, it's likely to be difficult or 
impossible to measure the speedup for many apps (on x86-32, SuSE don't ship 
code that uses direct seg refs because they measured negligible 
improvements).

There shouldn't be any performance harm in allowing direct seg refs on x86-64, 
though, since they don't need to be emulated as they do on x86-32.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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