Re: [PATCH] ppc32 dl-machine.c

[Geoff Keating <geoffk at geoffk dot org>]

> Date: Thu, 26 Sep 2002 03:19:44 +0200
> From: Segher Boessenkool <segher@koffie.nl>

> Geoff Keating wrote:
> > 
> > Segher Boessenkool <segher@koffie.nl> writes:
> > 
> > > Anton Blanchard wrote:
> > > > Looks good, except we should have a sync between dcbst and icbi. This
> > > > patch applies on top of the last.
> > > >
> > >
> > > You need a sync between icbi and isync, too.  See for example the MPC7410
> > > user manual, 2.3.4.3.4.
> > 
> > It's not necessary according to the architecture docs.  If the MPC7410
> > really needs it, it's broken.
> 
> Well, I read that doc as "this is a *typical* code sequence for *uni-processor*
> systems."
> 
> Also, from the specification of the sync and isync instructions, it's clear
> that the sync is needed.

You'll have to explain that, I don't see it.

> > > Oh, and I don't want to think about the races when
> > > the kernel decides to schedule in the middle of this sequence, and the
> > > next time this process is run, it runs on a different cpu...
> > 
> > The processors in a multiprocessor system broadcast these operations
> > on the bus, and the kernel is responsible for waiting long enough when
> > migrating a process to ensure that broadcasts have completed (it gets
> > this free since it needs to lock its own data structures anyway).  The
> > only operation here that doesn't get broadcast is 'isync'.
> 
> icbi isn't broadcast on all systems, either...

In that case, the kernel has to flush the whole icache when migrating
processes.

-- 
- Geoffrey Keating <geoffk@geoffk.org>