Re: [PATCH] powerpc/32: Move entry_32 functions just after HEAD functions.

[Christophe LEROY]

Le 20/04/2017 à 13:25, Michael Ellerman a écrit :
> David Laight <David.Laight@xxxxxxxxxx> writes:
>
> > From: Christophe Leroy
> > > By default, PPC8xx PINs an ITLB on the first 8M of memory in order
> > > to avoid any ITLB miss on kernel code.
> > > However, with some debug functions like DEBUG_PAGEALLOC and
> > > (soon to come) DEBUG_RODATA, the PINned TLB is invalidated soon
> > > after startup so ITLB missed start to happen also on the kernel code.
> > >
> > > In order to avoid any ITLB miss in a critical section, we have to
> > > ensure that their is no page boundary crossed between the setup of
> > > a new value in SRR0/SRR1 and the associated RFI. This cannot be done
> > > easily if entry_32 functions sits in the middle of other .text
> > > functions. By placing entry_32 just after the .head section (as already
> > > done for entry_64 on PPC64), we can more easily ensure the issue
> > > doesn't happen.
> >
> > Shouldn't this be done by putting all the functions that 'matter'
> > into a named section instead of relying on the order of the input files?
> > (Which is what I think this is doing.)
>
> Yeah that is fragile if there's nothing more to it.
>
> I'm not sure if we need a special section. If the functions that must
> not cross a page boundary are aligned to a page boundary (with .align)
> then that would also work wouldn't it?

Yes it would work, but we would also 'loose' some space (up to one page 
?), as I don't think 'ld' would fill the blanks with other functions.

Maybe we can find a lower alignment than a page boundary, I'll 
investigate that.

Christophe