[PATCH] arm64: set MITIGATE_SPECTRE_BRANCH_HISTORY to n by default

[Chanho Min]

lmbench shows 25% performance regression after MITIGATE_SPECTRE_BRANCH_HISTORY
is enabled. This performance drop may be more significant than mitigating
the spectre-BHB. This patch changes this security option to disable by default
and makes it selectable.

- lat_syscall result with MITIGATE_SPECTRE_BRANCH_HISTORY enabled (cortex-a78)
write call      0.2777  0.2810  0.2824  0.280367
read call       0.3435  0.3452  0.3443  0.344333

- lat_syscall result with MITIGATE_SPECTRE_BRANCH_HISTORY disabled (cortex-a78)
write call      0.2101  0.2117  0.2116  0.2111
read call       0.2732  0.2744  0.2763  0.274633

Signed-off-by: Chanho Min <chanho.min@xxxxxxx>
---
 arch/arm64/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5d91259ee7b5..be76f425c060 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1626,7 +1626,7 @@ config UNMAP_KERNEL_AT_EL0
 
 config MITIGATE_SPECTRE_BRANCH_HISTORY
 	bool "Mitigate Spectre style attacks against branch history" if EXPERT
-	default y
+	default n
 	help
 	  Speculation attacks against some high-performance processors can
 	  make use of branch history to influence future speculation.
-- 
2.17.1