linux-kernel - [PATCH AUTOSEL 4.19 21/21] powerpc/dcr: Use cmplwi instead of 3-argument cmpli

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Message-Id: <20211109222311.1235686-21-sashal@kernel.org>
Date:   Tue,  9 Nov 2021 17:23:10 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sasha Levin <sashal@...nel.org>, benh@...nel.crashing.org,
        paulus@...ba.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH AUTOSEL 4.19 21/21] powerpc/dcr: Use cmplwi instead of 3-argument cmpli

From: Michael Ellerman <mpe@...erman.id.au>

[ Upstream commit fef071be57dc43679a32d5b0e6ee176d6f12e9f2 ]

In dcr-low.S we use cmpli with three arguments, instead of four
arguments as defined in the ISA:

	cmpli	cr0,r3,1024

This appears to be a PPC440-ism, looking at the "PPC440x5 CPU Core
User’s Manual" it shows cmpli having no L field, but implied to be 0 due
to the core being 32-bit. It mentions that the ISA defines four
arguments and recommends using cmplwi.

It also corresponds to the old POWER instruction set, which had no L
field there, a reserved bit instead.

dcr-low.S is only built 32-bit, because it is only built when
DCR_NATIVE=y, which is only selected by 40x and 44x. Looking at the
generated code (with gcc/gas) we see cmplwi as expected.

Although gas is happy with the 3-argument version when building for
32-bit, the LLVM assembler is not and errors out with:

  arch/powerpc/sysdev/dcr-low.S:27:10: error: invalid operand for instruction
   cmpli 0,%r3,1024; ...
           ^

Switch to the cmplwi extended opcode, which avoids any confusion when
reading the ISA, fixes the issue with the LLVM assembler, and also means
the code could be built 64-bit in future (though that's very unlikely).

Reported-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
BugLink: https://github.com/ClangBuiltLinux/linux/issues/1419
Link: https://lore.kernel.org/r/20211014024424.528848-1-mpe@ellerman.id.au
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/powerpc/sysdev/dcr-low.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/sysdev/dcr-low.S b/arch/powerpc/sysdev/dcr-low.S
index e687bb2003ff0..5589fbe48bbdc 100644
--- a/arch/powerpc/sysdev/dcr-low.S
+++ b/arch/powerpc/sysdev/dcr-low.S
@@ -15,7 +15,7 @@
 #include <asm/export.h>

 #define DCR_ACCESS_PROLOG(table) \
-	cmpli	cr0,r3,1024;	 \
+	cmplwi	cr0,r3,1024;	 \
 	rlwinm  r3,r3,4,18,27;   \
 	lis     r5,table@h;      \
 	ori     r5,r5,table@l;   \
-- 
2.33.0