Re: [RFC Patch]Use ar.kr2 for smp_processor_id

[Zou Nan hai <nanhai.zou@intel.com>]

On Thu, 2007-02-08 at 14:37, Keith Owens wrote:
> Zou Nan hai (on 08 Feb 2007 12:27:31 +0800) wrote:
> >On Thu, 2007-02-08 at 14:04, Keith Owens wrote:
> >> Zou Nan hai (on 08 Feb 2007 11:28:44 +0800) wrote:
> >> >Pin ar.kr2 of each CPU, so that smp_processor_id can use it.
> >> 
> >> Historically ar.k2 has been reserved for debugging purposes, for
> >> example in ivt.S.  Debuggers often need a location that can be used
> to
> >> track progress, it has to be somewhere that does not rely on TLB
> >> entries and is guaranteed to appear in MCA/INIT records - ar.k2 is
> >> perfect for this.
> >> 
> >  Ok, seems that current kr3 is only used by ia64_itc_printk_clock?
> >> Use Tony's suggestion of testing for a change in ar.k3 (guaranteed
> to
> >> be unique on every cpu) and caching the corresponding cpu number
> when
> >> it changes.
> >> 
> >  But why do we even need to cache it? 
> >
> >  It is already in a register if we put it to kr3. 
> >  so smp_processor_id() could be very fast. and later sys_getcpu can
> >also be very fast.
> 
> ar.k3 is currently used for the address of the per-cpu data area,
> which
> speeds up access to all the per-cpu data.  Changing ar.k3 to hold the
> cpu number means an extra array calculation and lookup for every
> per-cpu variable, slowing down the rest of the system.
> 

  Are you sure ar.k3 is used by per-cpu data access? Disassembly of
vmlinux show there are only MCA code and ia64_itc_printk_clock used
ar.k3.

Thanks
Zou Nan hai