[PATCH] x86_64 irq: Allocate a vector across all cpus for genapic_flat.

The problem we can't take advantage of lowest priority delivery mode if
the vectors are allocated for only one cpu at a time.  Nor can we work
around hardware that assumes lowest priority delivery mode is always
used with several cpus.

So this patch introduces the concept of a vector_allocation_domain.  A
set of cpus that will receive an irq on the same vector.  Currently the
code for implementing this is placed in the genapic structure so we can
vary this depending on how we are using the io_apics.

This allows us to restore the previous behaviour of genapic_flat without
removing the benefits of having separate vector allocation for large
machines.

This should also fix the problem report where a hyperthreaded cpu was
receving the irq on the wrong hyperthread when in logical delivery mode
because the previous behaviour is restored.

This patch properly records our allocation of the first 16 irqs to the
first 16 available vectors on all cpus.  This should be fine but it may
run into problems with multiple interrupts at the same interrupt level.
Except for some badly maintained comments in the code and the behaviour
of the interrupt allocator I have no real understanding of that problem.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Muli Ben-Yehuda <muli@il.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>