Re: proposed interface change for setting the ldt

[Jeremy Fitzhardinge <jeremy@goop.org>]

Zachary Amsden wrote:
> This interface doesn't work for anything other than Xen.

It works OK for native.  It's a very simple rolling together of two 
operations which always happen together anyway.

> It is impossible to implement it without specific knowledge of kernel 
> internals, since it doesn't provide the GDT selector for the LDT.

Neither does the previous interface.  load_ldt_desc needs to have the 
specific LDT entry hardcoded into it.

>   Now everything that looks like real hardware needs to move the 
> knowledge of the LDT structure into paravirt-ops, 

Do you mean the GDT structure?

> and it has no clear calling convention, so you've now got to reason 
> about SMP preemption correctness inside the paravirt-op, instead of at 
> the higher level where it should be done.

The previous interface already required that preempt be disabled around 
those functions.  In the previous interface, set_ldt_desc takes a cpu 
number, but it is required to equal the current cpu; load_ldt_desc 
always operates on the current CPU.  It therefore requires that those 
two ops be paired with preempt disabled.  The new interface is simpler, 
but still requires preempt disabled around it.

In general, the set_ldt interface is cleaner for the base kernel, and 
much cleaner for Xen, while being trivial to implement for native 
hardware or something which looks like it.

    J