linux-kernel - [PATCH v2] powerpc: warn on emulation of dcbz instruction in kernel mode

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Message-Id: <2e3acfe63d289c6fba366e16973c9ab8369e8b75.1631803922.git.christophe.leroy@csgroup.eu>
Date:   Thu, 16 Sep 2021 16:52:09 +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 v2] powerpc: warn on emulation of dcbz instruction in kernel mode

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 in kernel mode due to dcbz, so that the user is made aware that
dcbz instruction has been used unexpectedly by the kernel.

Reported-by: Stan Johnson <userm57@...oo.com>
Cc: Finn Thain <fthain@...ux-m68k.org>
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
v2: Warn only when emulating kernel mode
---
 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..bf96b954a4eb 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(!user_mode(regs));
 		r = emulate_dcbz(op.ea, regs);
 	} else {
 		if (type == LARX || type == STCX)
-- 
2.31.1