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

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Date:   Thu, 16 Sep 2021 17:15:16 +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 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 ?

Ben.