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Re: [Xen-users] Debian XenSpecificGlibc

Why would I want a specialized glibc? What is wrong with the standard one?


first of all, this is only interessting for i386. amd64 (and probably other architectures) doesn't have this problem at all.

I wouldn't expect it to be a problem for anything but 32 bit x86. The problem is the glibc exploits a - rather bizarre - feature of the x86 segmentation hardware.

Because we don't use the segmentation hardware to protect Xen on x86_64, it's not a problem on that architecture. AFAIK, neither Xen nor glibc use nasty segmentation features on other architectures (I'm not even sure that the other architectures we support actually have rich enough - or any - segmentation features for this to work in any case) will do something different again, so it shouldn't be a problem there either.


With the std. libc6 package you need to move /lib/tls away on i386 xen hosts, because otherwise you would loose a lot of performance. I think every user already saw xen warning about that at boot time.

but moving /lib/tls away is not optimal for more than one reason: - it's no permanent solution. You have to do this after each glibc upgrade on each dom0 and domU. - if you switch a lot between a non-xenified kernel and the xen hypervisor+kernel you will end up in moving the /lib/tls directory a lot. - moving /lib/tls away disabled more features than needed. java and some other programs will lose performance because of /lib/tls is missing completly. - this solution isn't user-friendly at all.

With the glibc packages by Aurelien Jarno (part of the official debian-glibc team) all these problems will be fixed.

You have to install "libc6-xen" once, but afterwards never need to take care of this again.

- if no xen kernel is running the default libc6 is used like libc6-xen never had been installed. - when a xen kernel is running then the xen optimized version is used for optimal performance without the user have to do something. with this special libc6 package java, mysql and other applications should benifit and have a better performance - this setup will not break on libc6 upgrades - it works in dom0 and domU's. - this solution will be "officially" supported by debian (if xen3 and these packages are officially released in debian). you can fill bug reports if something is not working as expected...

I think (if no big problems are found) this will go in debian unstable with the next glibc upgrade.

But for now, these libc6 packages are not tested! Everybody may use them, but without any promise that it is really working as expected... Some team members of the pkg-xen debian group (including me) will test these libc6 packages in the next days.


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