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xen-devel
Re: [Xen-devel] Re: "Barry Silverman": Setting GDT entries for Thread Lo
> I don't want to throw a monkeywrench in your plans, but another
> potential trouble spot is the x86 vsyscall interface in 2.6. The
> vsyscall region sits at the top of the virtual address space, and could
> conflict with the Xen mapping. You may have to consider remapping Xen
> to another memory region, but I'm not sure where there may be other
> trouble areas with Windows domains (map?). At some point in the
> forseeable future, it might no longer be possible to locate Xen at an OS
> neutral location, so perhaps it is worth considering the remapping
> problem now.
The vsyscall interface happens to sit at the top of virtual memory in
native Linux, I think mainly because the vsyscall page is allocated
using the Linux 'fixmap' mechanism.
However, the address of the vsyscall interface is passed to
applications as part of the ELF-loading protocol. It should therefore
be possible to map the vsyscall stubs to a different virtual address
in Xenolinux (Fingers crossed!).
I hope we don't have to go down a 'map Xen to guest-specific location'
kind of route. I think that position-independent code in x86 is likely
to perform below-par (no PC-relative addressing; allocating a base
register is painful on a register-starved architecture).
-- Keir
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