Re: [Xen-devel] segmentation and Xen

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 14 Oct 2005, at 18:00, Jonathan M. McCune wrote:

> We noticed a difference in the kernel and user space code and data 
> segment descriptor entries in the GDTs for Xen and XenoLinux.  It does 
> not appear that the Xen GDT totally supplants the Linux GDT, as the 
> Linux GDT has its limit changed (in the descriptor, but not the 
> comments) appropriately to make room for Xen in the upper 64 MB.  The 
> difference is in the "Accessed" bit of the "Type" field, as defined in 
> Chapter 4, Volume 3, of the Intel manuals.  Can you help us to 
> understand why the Access bit is set in the Linux kernel code but not 
> in the Xen code?
Noone uses the accessed bit for anything, so it doesn't matter how it 
is set. I guess if you set it in the table definition then the CPU 
doesn't need to set the accessed bit the first time the segment is 
loaded, but that really doesn't matter either way...
> More generally, how do the GDTs defined in head.S and x86_32.S 
> interact?  It seems problematic that Xen defines a GDT for guest OSes, 
> but guest OSes are allowed to retain a GDT of their own.
Note that the Xen GDT (defined in x86_32.S) is not based at offset 
zero! Xen reserves the final eighth of the GDT space. The first 
seven-eighths are available for guests to define.
 -- Keir


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