Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp_* memory barriers

From: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To: Paul Burton <paul.burton@imgtec.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>,
	linux-mips@linux-mips.org, benh@kernel.crashing.org,
	will.deacon@arm.com, linux-kernel@vger.kernel.org,
	ralf@linux-mips.org, markos.chandras@imgtec.com,
	macro@linux-mips.org, Steven.Hill@imgtec.com,
	alexander.h.duyck@redhat.com, davem@davemloft.net
Subject: Re: [PATCH 1/3] MIPS: R6: Use lightweight SYNC instruction in smp_* memory barriers
Date: Tue, 2 Jun 2015 14:12:29 +0200	[thread overview]
Message-ID: <20150602121227.GA1474@macpro.local> (raw)
In-Reply-To: <20150602100835.GG24014@NP-P-BURTON>

On Tue, Jun 02, 2015 at 11:08:35AM +0100, Paul Burton wrote:
> Hi Leonid,
> 

<snip>

> > +
> > +	  If that instructions are not implemented in processor then it is
> > +	  converted to generic "SYNC 0".
> 
> I think this would read better as something like:
> 
>   If a processor does not implement the lightweight sync operations then
>   the architecture requires that they interpret the corresponding sync
>   instructions as the typical heavyweight "sync 0". Therefore this
>   should be safe to enable on all CPUs implementing release 2 or
>   later of the MIPS architecture.
> 

Is it really the case for release 2?

I'm asking because recently I needed to do something similar and I couldn't
find this garantee in the revision 2.00 of the manual.
May it's just poorly formulated but here is what I find in it:
- "The stype values 1-31 are reserved for future extensions to the architecture."
  (ok)
- "A value of zero will always be defined such that it performs all defined
   synchronization operations." (ok)
- "Non-zero values may be defined to remove some synchronization operations."
  (ok, certainly if we understand the word "weaker" instead of "remove")
- "As such, software should never use a non-zero value of the stype field, as
  this may inadvertently cause future failures if non-zero values remove
  synchronization operations." (Mmmm, ok but ...)
Nowhere is there something close to what is found in the revision 5.0 or later:
  "If an implementation does not use one of these non-zero values to define a
   different synchronization behavior, then that non-zero value of stype must
   act the same as stype zero completion barrier."

The wording may have changed since revision 2.8 but I don't have access to the
corresponding manual.

Luc Van Oostenryck