Gerd Knorr wrote:

> i.e. something like this (as basic idea, patch is far away from doing 
> anything useful ...)?

Adapting $subject to the actual topic, so other lkml readers can catch up ;)

Ok, here new version of the SMP alternatives patch.  It features:

  * it actually compiles and boots, so you can start playing with it ;)
  * reuses the alternatives bits we have already as far as possible.
  * separate table for the SMP alternatives, so we can keep them and
    switch at runtime between SMP and UP (for virtual CPU hotplug).
  * two new alternatives macros, one generic which can handle quite
    comples stuff such as spinlocks, one for the "lock prefix" case.

TODO list:

  * convert more code (bitops, ...).
  * module support (using modules is fine, they run the safe SMP
    version of the code, they just don't benefit from the optimizations
    yet).
  * integrate with xen bits and CPU hotplug, at the moment it's a
    boot-time only thing.
  * benchmark it.
  * x86_64 version.
  * drop the printk's placed into the code for debugging.
  * probably more ...

How it works right now:

  * The patch switches to UP unconditionally when doing the usual
    alternatives stuff at boot time
  * Just before booting the second CPU it switches to SMP.

How to test:

  * boot with "maxcpus=1" to run the UP code.

Comments are welcome.

cheers,

   Gerd