linux-kernel - [RFC 2/3] arm: enable 64BIT_ATOMIC(_ALIGNED)

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Date:   Thu, 24 Aug 2017 14:42:56 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Michal Marek <mmarek@...e.com>,
        Laura Abbott <labbott@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>, mic@...ikod.net,
        Russell King <linux@...linux.org.uk>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Jonathan Austin <jonathan.austin@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        hoeun.ryu@...il.com, linux-arch@...r.kernel.org
Subject: [RFC 2/3] arm: enable 64BIT_ATOMIC(_ALIGNED)_ACCESS on LPAE enabled machines

 'ldrd/strd' (load/store doublewords) instructions are 64bit atomic as
long as the address is 64-bit aligned on LPAE (Large Physical Address
Extension) enabled architectures. This feature is to guarantee atomic
accesses on newly introduced 64bit wide descriptors in the translation
tables.

 Making 64BIT_ATOMIC_ACCESS true, some kernel codes to access 64bit
variables can be optimized by omitting seqlock or the mimic of it.
 Also make 64BIT_ATOMIC_ALIGNED_ACCESS true, the 64bit atomic access is
guarnteed only when the address is 64bit algined.

Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
---
 arch/arm/mm/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/mm/Kconfig b/arch/arm/mm/Kconfig
index 60cdfdc..3142572 100644
--- a/arch/arm/mm/Kconfig
+++ b/arch/arm/mm/Kconfig
@@ -660,6 +660,8 @@ config ARM_LPAE
 	bool "Support for the Large Physical Address Extension"
 	depends on MMU && CPU_32v7 && !CPU_32v6 && !CPU_32v5 && \
 		!CPU_32v4 && !CPU_32v3
+	select 64BIT_ATOMIC_ACCESS
+	select 64BIT_ATOMIC_ALIGNED_ACCESS
 	help
 	  Say Y if you have an ARMv7 processor supporting the LPAE page
 	  table format and you would like to access memory beyond the
-- 
2.7.4