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

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Date:   Wed, 15 Sep 2021 16:31:40 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Stan Johnson <userm57@...oo.com>,
        Finn Thain <fthain@...ux-m68k.org>
Subject: [PATCH] powerpc: warn on emulation of dcbz instruction

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);
 		r = emulate_dcbz(op.ea, regs);
 	} else {
 		if (type == LARX || type == STCX)
-- 
2.31.1