[Xen-devel] Re: [RFC, PATCH 7/24] i386 Vmi memory hole

[Chris Wright <chrisw@xxxxxxxxxxxx>]

* Zachary Amsden (zach@xxxxxxxxxx) wrote:
> Chris Wright wrote:
> >* Zachary Amsden (zach@xxxxxxxxxx) wrote:
> >  
> >>Allow creation of an compile time hole at the top of linear address space.
> >>
> >>Extended to allow a dynamic hole in linear address space, 7/2005.  This
> >>required some serious hacking to get everything perfect, but the end 
> >>result
> >>appears to function quite nicely.  Everyone can now share the appreciation
> >>of pseudo-undocumented ELF OS fields, which means core dumps, debuggers
> >>and even broken or obsolete linkers may continue to work.
> >>    
> >
> >Thanks.  Gerd did something similar (although I believe it's simpler,
> >don't recall the relocation magic) for Xen.  Either way, it's useful
> >from Xen perspective.
> 
> I believe Xen disables sysenter.

Yes, so vsyscall page has int80 implementation.

> The complications in my patch come 
> from the fact that the vsyscall page has to be relocated dynamically, 
> requiring, basically run time linking on the page and some tweaks to get 
> sysenter to work.  If you don't use vsyscall (say, non-TLS glibc), then 
> you don't need that complexity.  But I think it might be needed now, 
> even for Xen.

I believe both Xen and execshield move vsyscall out of fixmap, and then
map into userspace as normal vma.

thanks,
-chris

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