proposed interface change for setting the ldt

[Jeremy Fitzhardinge <jeremy@goop.org>]

At the moment, all the places where the LDT are set in the kernel are of 
the form:

    set_ldt_desc(cpu, segments, count);
    load_LDT_desc();

(two instances in asm-i386/desc.h).  set_ldt_desc() sets an LDT 
descriptor in the GDT, and load_LDT_desc() is basically just lldt. These 
map to the write_gdt_entry and load_ldt_desc paravirt ops.

This doesn't work well for Xen, because you set the ldt directly by 
passing the base+size into the hypervisor.  In fact, it doesn't allow 
you to set an LDT-type descriptor into the GDT, so this current 
interface requires the Xen backend to decode the descriptor passed to 
write_gdt_entry, look to see if its an LDT; if so, store the base+size 
somewhere, and then when load_ldt_desc() is called, do the appropriate 
Xen hypercall.

A better interface for us would be simply:

	set_ldt(const struct desc_struct *ldt, int num_entries);

since this maps directly to the appropriate Xen hypercall.  If you still 
want to implement it by plugging the LDT descriptor into the GDT and 
then lldt, then there's no reason you can't implement it that way.

Thoughts?

    J