Re: i386 Xen PV support still needed?

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On Tue, Aug 01, 2017 at 11:38:45PM +0200, Dominik 'Rathann' Mierzejewski wrote:
> On Tuesday, 01 August 2017 at 14:19, Florian Weimer wrote:
> > We still build a special glibc variant for Xen which avoids certain
> > segment-relative accesses which are difficult to emulate with
> > paravirtualization..
> > 
> > Is this still needed?  Can we drop it?

This is a sort of historical feature.  I had only a vague memory of
this from when I last used Xen PV & 32 bit guests (which must have
been around 2005?), but the precise problem and solution is explained
in these pages on the old XenSource wiki:

https://web.archive.org/web/20080421123834/http://wiki.xensource.com:80/xenwiki/XenSegments
https://web.archive.org/web/20080218095938/wiki.xensource.com/xenwiki/XenSpecificGlibc

Note the binaries for Fedora Core 3 :-)

> What is the performance difference between running a regular glibc under
> Xen vs. this special one? I believe there may still be some value in
> running Fedora in a Xen i686 guest VM.

I seem to recall that the performance penalty was very significant.

However, note that this is only for using Xen paravirtualization.
In no way would dropping this prevent you from using Xen or 32 bit
i686 guests, but you would have to use full virtualization.  That
requires hardware virt, but (almost literally) every Intel and AMD
processor since 2006 has shipped with hardware virt.

Also this doesn't apply to 64 bit guests at all (even PV) since they
didn't have to do fun things with segment registers to protect the
hypervisor from the guest.

Rich.

-- 
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