linux-kernel - Re: [PATCH] powerpc: warn on emulation of dcbz instruction

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Message-ID: <eb1a39368401bf46e805ca64256604cc649f771e.camel@kernel.crashing.org>
Date:   Thu, 16 Sep 2021 17:16:30 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Stan Johnson <userm57@...oo.com>,
        Finn Thain <fthain@...ux-m68k.org>
Subject: Re: [PATCH] powerpc: warn on emulation of dcbz instruction

On Thu, 2021-09-16 at 17:15 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2021-09-15 at 16:31 +0200, Christophe Leroy wrote:
> > dcbz instruction shouldn't be used on non-cached memory. Using
> > it on non-cached memory can result in alignment exception and
> > implies a heavy handling.
> > 
> > Instead of silentely emulating the instruction and resulting in
> > high
> > performance degradation, warn whenever an alignment exception is
> > taken due to dcbz, so that the user is made aware that dcbz
> > instruction has been used unexpectedly.
> > 
> > Reported-by: Stan Johnson <userm57@...oo.com>
> > Cc: Finn Thain <fthain@...ux-m68k.org>
> > Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> > ---
> >  arch/powerpc/kernel/align.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/powerpc/kernel/align.c
> > b/arch/powerpc/kernel/align.c
> > index bbb4181621dd..adc3a4a9c6e4 100644
> > --- a/arch/powerpc/kernel/align.c
> > +++ b/arch/powerpc/kernel/align.c
> > @@ -349,6 +349,7 @@ int fix_alignment(struct pt_regs *regs)
> >  		if (op.type != CACHEOP + DCBZ)
> >  			return -EINVAL;
> >  		PPC_WARN_ALIGNMENT(dcbz, regs);
> > +		WARN_ON_ONCE(1);
> 
> This is heavy handed ... It will be treated as an oops by various
> things uselessly spit out a kernel backtrace. Isn't
> PPC_WARN_ALIGNMENT
> enough ?

Ah I saw your other one about fbdev...  Ok what about you do that in a
if (!user_mode(regs)) ?

Indeed the kernel should not do that.

Cheers,
Ben.