| Date | Thu, 29 Jan 2009 23:29:56 +0100 | From | Ingo Molnar <> | Subject | Re: x86: unify genapic code, unify subarchitectures, remove old subarchitecture code |
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* 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
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