Proposed interface change

[Zachary Amsden <zach@vmware.com>]

I would like to propose an interface change to the following 
paravirt-ops calls:

        void (fastcall *write_ldt_entry)(void *dt, int entrynum, u64 entry);
        void (fastcall *write_gdt_entry)(void *dt, int entrynum, u64 entry);
        void (fastcall *write_idt_entry)(void *dt, int entrynum, u64 entry);


Can we consolidate the dt and entrynum parameters and just pass 
dt+entrynum*8?  I don't know if this makes things harder for the Xen 
case, but I seem to distantly recall that we used to pass a pure pointer 
- actually we used to have a pure post-update call, but changed the 
interface to accommodate Xen.  When we did that, the interface changed 
to take the new descriptor value as a parameter, as a raw write to the 
GDT / LDT would not be allowed.  And during that step, the new entrynum 
parameter was added.

Since it's fairly easy to merely subtract the entry offset from the 
descriptor base, and descriptor tables are virtually mapped, obtaining 
the page number, if it is required, is still a very easy task.  I 
believe we added the entrynum field so the page number could be 
computed, but I just don't remember why, and don't see any reason for it 
now.

So I propose changing the convention here to eliminate the entrynum 
field, thus allowing all the parameters to fit within the fastcall 
convention, which affords us three registers.  The primary motivation 
being, the call to write_gdt_entry is on the fast path in context switch.

Any objections?

Zach