linux-kernel - Re: [RFC PATCH] powerpc: Convert ____flush_dcache_icache

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Message-ID: <20190902111105.GX31406@gate.crashing.org>
Date:   Mon, 2 Sep 2019 06:11:05 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     "Alastair D'Silva" <alastair@....ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH] powerpc: Convert ____flush_dcache_icache_phys() to C

On Mon, Sep 02, 2019 at 11:48:59AM +1000, Michael Ellerman wrote:
> "Alastair D'Silva" <alastair@....ibm.com> writes:
> > On Wed, 2019-08-21 at 22:27 +0200, Christophe Leroy wrote:
> >> Can we be 100% sure that GCC won't add any code accessing some
> >> global data or stack while the Data MMU is OFF ?
> >
> > +mpe
> >
> > I'm not sure how we would go about making such a guarantee, but I've
> > tied every variable used to a register and addr is passed in a
> > register, so there is no stack usage, and every call in there only
> > operates on it's operands.
> 
> That's not safe, I can believe it happens to work but the compiler
> people will laugh at us if it ever breaks.

Yes.  Sorry.

> Let's leave it in asm.

+1

The asm is simpler, more readable, more maintainable, and perhaps more
performant even.  Plus the being-laughed-at issue.


Segher