Re: x86: unify genapic code, unify subarchitectures, remove old subarchitecture code

[Ingo Molnar <mingo@elte.hu>]

* Andi Kleen <andi@firstfloor.org> wrote:

> Also bigsmp is kind of obsolete too, it could be probably
> merged with default with very tiny impact because it's not
> all that different.

i dont think you have understood the driving principle behind this 
restructuring and unification.

The whole _point_ of the apic driver infrastructure is to use it for 
dissimilar APIC modes.

In the future we'll create _more_ of them, not less - so that we can 
remove runtime conditionals from the drivers and that we can use the APIC 
driver for the hardware that is the most efficient.

For example, the apic_default driver (which is used if all APIC IDs are 
between 0 and 7 - which is the majority of current hardware), is more 
efficient at sending IPIs, because it can send a broadcast IPI (used for 
TLB flushes for example) as a single IPI.

The apic_bigsmp driver on the other hand has to use a 'serialized' 
sequence of IPIs, to each CPU separately. That is less efficient, 
especially for something as performance-sensitive as TLB flushes.

To "unify" the ability of apic_bigsmp to deal with APIC IDs >= 8 with the 
more efficient logical addressing mode of apc_default is possible but ugly 
- we'd have to add runtime checks and flip between the two methods. What 
So what you call "very tiny impact" is in reality unnecessary and 
avoidable overhead.

	Ingo