[Pkg-xen-devel] Re: Xen and glibc

[Aurelien Jarno]

Julien Danjou a écrit :
> Hello,
Hi!

> We [the Debian Xen package team] are currently working on Xen packages,
> and are planning to include them into Debian.
> 
> A current issue in Xen, is the libc problem.
> 
> From the Xen wiki [1]:
> "Xen uses segmentation to provide protection of the memory used for the
> hypervisor. This results in some performance issues since wrap-around
> segments as used by glibc need expensive extra handling. [...]
> 
> It is possible to rebuild glibc so that it only uses segments such that
> there is no performance penalty. To do this, you need to apply the patch
> below to the glibc sources and then rebuild glibc with the
> -mno-tls-direct-seg-refs option."
> 
> Currently, we expect our users to move /lib/tls away (or to create
> /etc/ld.so.hwnocap, thanks to aurel32 for the tips).
> You will understand that this is not very convenient, and will disable
> more stuff than really needed.
> 
> Would it possible to add an extra flavor to the current glibc with the
> correct build options ?
> 
> Please note that this issue is only available for i386 arch.
> 

As already said on IRC, my main concern is that if we accept that, it 
will be more difficult to refuse some more flavors. I don't want to end 
up with 10 flavors of the glibc. If we stop to Xen, that's ok for me.

On the technical points::
- The patch is not conditional, and it is currently not possible to use 
different sources for different flavors. But as it is fixed in glibc 
2.4, it should be possible to backport the fixes.
- How we detect to use this flavor and not the tls or the default one? 
Is there any flag exported by the kernel? How is it done on other 
distributions?

Cheers,
Aurelien

-- 
   .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
  : :' :  Debian developer           | Electrical Engineer
  `. `'   aurel32 at debian.org         | aurelien at aurel32.net
    `-    people.debian.org/~aurel32 | www.aurel32.net