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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> I am trying to port the Fedora version of Linux to xeno-linux. It involves
> porting the NPTL Library (New Posix Thread Library), and TLS (Thread Local
> Storage) kernel changes which Redhat backported from 2.6 into 2.4.
> 
> It seems that with TLS the guest Fedora must:
> 1) Have a per-cpu GDT
> 2) Be able to change some descriptors within the GDT when the
> "set_thread_area" syscall is executed, and on thread context switches. TLS
> maintain a small array of free descriptors which can be freely used by for
> thread local storage by the guest libc - (mediated by the guest syscall)
> 
> In looking at the code, I have seen how a Xen can set up its own per CPU GDT
> when Domain 0 is building it, but how would a non-Domain 0 guest change GDT
> descriptors on the fly for "set_thread_area"?

You would use HYPERVISOR_update_descriptor(), which takes the physical
address of the descriptor-table entry to be updated, and the two
32-bit values to write into that descriptor entry.

This shoudl all work straightforwardly, unless TLS tries to install
segments that are "full size" (i.e. 4GB). I know that one of the
recent glibc threading solutions tries to do this --- it won't work
with Xen because we clip segments so that they do not overlap with
Xen's reserved area in the top 64MB of the virtual address space.

 -- Keir


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