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Re: [PATCH] ppc32 dl-machine.c

Segher Boessenkool <> 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,

It's not necessary according to the architecture docs.  If the MPC7410
really needs it, it's broken.

> Even this won't work on all (multiprocessor) machines; dcbf or similar is
> necessary there.  

I believe no-one has made a chip that requires this.  I hope no-one
ever does, because of the additional performance hit of going to main
memory instead of L2 cache.

> 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'.

- Geoffrey Keating <>

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