Loading handle_arch_irq with a PC relative load

[Nicolas Pitre]

On Fri, 13 Jul 2012, Gilles Chanteperdrix wrote:

> On 07/13/2012 09:40 PM, Nicolas Pitre wrote:
> > On Fri, 13 Jul 2012, Gilles Chanteperdrix wrote:
> > 
> >>
> >> I do not know if it is really useful, but it seems it would be possible 
> >> to reduce the number of memory accesses to just one in the irq_handler 
> >> macro in the case where CONFIG_MULTI_IRQ_HANDLER is enabled, by using a 
> >> PC relative load, with something like the following patch:
> > 
> > To be strict with ccode sections, you can't do this.  The 
> > handle_arch_irq symbol identifies a variable and with your patch you're 
> > moving it from the .data section to the .text section.  The .text 
> > section is meant to be read only, and this is even more true when using 
> > a XIP kernel where .text is in ROM, or if we could make the access 
> > protection of the kernel ro.
> 
> I understand that but, XIP kernel aside, the handle_arch_irq variable is
> set only once very early during the boot process, so, almost read-only.
> Is not Linux using self-modifying code in some cases anyway (booting an
> SMP kernel on an UP processor for instance).

There are limits to which such tricks should be applied.  In the SMP on 
UP case this is a matter of making the kernel boot at all which is a 
rather strong reason.

Do you have performance numbers like interrupt latency that show this 
patch being worth it?  Without concrete justifications I don't think we 
should go down that path.


Nicolas