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Re: [Xen-devel] Re: "Barry Silverman": Setting GDT entries for Thread Local Storage

Keir Fraser wrote:

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 think that might work, but it seems somewhat unnecessary. Does Xen use the very top part of VM space for any special purpose - e.g. stack? Why not just make Xen itself a kernel fixmap entry to avoid collisions?

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).
Not to mention having to deal with relocatable pointers across different domains! Ugh.

Zachary Amsden

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