linux-kernel - Re: [PATCH v2] arch/powerpc/include/asm/barrier.h: redefine rmb and wmb to lwsync

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Date:   Thu, 23 Feb 2023 20:24:34 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Kautuk Consul <kconsul@...ux.vnet.ibm.com>
Cc:     paulmck@...nel.org, Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Rohan McLure <rmclure@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arch/powerpc/include/asm/barrier.h: redefine rmb and
 wmb to  lwsync

Kautuk Consul <kconsul@...ux.vnet.ibm.com> writes:
>  
>> You are correct, the patch is wrong because it fails to account for IO
>> accesses.
>
> Okay, I looked at the PowerPC ISA and found:
> "The memory barrier provides an ordering function for the storage accesses
> caused by Load, Store,and dcbz instructions that are executed by the processor
> executing the sync instruction and for which the specified storage location
> is in storage that is Memory Coherence Required and is neither Write Through
> Required nor Caching Inhibited.

Yep, that's the key sentence there. If you look at the definition for
"sync" it has not exceptions for different storage types.

I agree it's not very clear unless you're looking closely.

> Thanks for your time, Michael. Sorry for the noise.
>> 
>> Kautuk, I'm not sure what motivated you to look at these barriers, was
>> it just the documentation you linked to?

> I read the basic documentation. Now that I have access to the PowerISA
> document I guess I'll go through it more thoroughly.

The ISA is available publicly. There's links to most versions here:

https://wiki.raptorcs.com/wiki/Power_ISA

cheers