linux-kernel - [PATCH AUTOSEL 4.19 064/101] arm64: assembler: Switch ESB-instruction with a vanilla nop if !ARM64_HAS

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Date:   Fri, 19 Jul 2019 00:06:55 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 4.19 064/101] arm64: assembler: Switch ESB-instruction with a vanilla nop if !ARM64_HAS_RAS

From: James Morse <james.morse@....com>

[ Upstream commit 2b68a2a963a157f024c67c0697b16f5f792c8a35 ]

The ESB-instruction is a nop on CPUs that don't implement the RAS
extensions. This lets us use it in places like the vectors without
having to use alternatives.

If someone disables CONFIG_ARM64_RAS_EXTN, this instruction still has
its RAS extensions behaviour, but we no longer read DISR_EL1 as this
register does depend on alternatives.

This could go wrong if we want to synchronize an SError from a KVM
guest. On a CPU that has the RAS extensions, but the KConfig option
was disabled, we consume the pending SError with no chance of ever
reading it.

Hide the ESB-instruction behind the CONFIG_ARM64_RAS_EXTN option,
outputting a regular nop if the feature has been disabled.

Reported-by: Julien Thierry <julien.thierry@....com>
Signed-off-by: James Morse <james.morse@....com>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/arm64/include/asm/assembler.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
index f90f5d83b228..5a97ac853168 100644
--- a/arch/arm64/include/asm/assembler.h
+++ b/arch/arm64/include/asm/assembler.h
@@ -112,7 +112,11 @@
  * RAS Error Synchronization barrier
  */
 	.macro  esb
+#ifdef CONFIG_ARM64_RAS_EXTN
 	hint    #16
+#else
+	nop
+#endif
 	.endm

 /*
-- 
2.20.1