Re: [Xen-users] Debian XenSpecificGlibc

["M.A. Williamson" <maw48@xxxxxxxxx>]

> > Why would I want a specialized glibc? What is wrong with the standard 
> > one?
> > Dirk
> 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.
Cheers,
Mark

> 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.
> --Ralph
>
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