On Sat, Jun 19, 2021 at 11:35:34AM +0200, Christophe Leroy wrote:
>
>
> Le 18/06/2021 à 19:26, Mathieu Desnoyers a écrit :
> >----- On Jun 18, 2021, at 1:13 PM, Christophe Leroy
> >christophe.leroy@xxxxxxxxxx wrote:
> >[...]
> >>
> >>I don't understand all that complexity to just replace a simple
> >>'smp_mb__after_unlock_lock()'.
> >>
> >>#define smp_mb__after_unlock_lock() smp_mb()
> >>#define smp_mb() barrier()
> >># define barrier() __asm__ __volatile__("": : :"memory")
> >>
> >>
> >>Am I missing some subtility ?
> >
> >On powerpc CONFIG_SMP, smp_mb() is actually defined as:
> >
> >#define smp_mb() __smp_mb()
> >#define __smp_mb() mb()
> >#define mb() __asm__ __volatile__ ("sync" : : : "memory")
> >
> >So the original motivation here was to skip a "sync" instruction whenever
> >switching between threads which are part of the same process. But based on
> >recent discussions, I suspect my implementation may be inaccurately doing
> >so though.
> >
>
> I see.
>
> Then, if you think a 'sync' is a concern, shouldn't we try and remove the
> forest of 'sync' in the I/O accessors ?
>
> I can't really understand why we need all those 'sync' and 'isync' and
> 'twi' around the accesses whereas I/O memory is usually mapped as 'Guarded'
> so memory access ordering is already garantied.
>
> I'm sure we'll save a lot with that.
The point of the twi in the I/O accessors was to make things easier to
debug if the accesses fail: for the twi insn to complete the load will
have to have completed as well. On a correctly working system you never
should need this (until something fails ;-) )
Without the twi you might need to enforce ordering in some cases still.
The twi is a very heavy hammer, but some of that that gives us is no
doubt actually needed.
Segher
