linux-kernel - [RFCv1 2/4] arm64: entry: Always apply workaround for contextidr

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Date:   Thu, 21 Oct 2021 21:45:28 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nicholas Piggin <npiggin@...il.com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>, Joey Gouly <joey.gouly@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        James Clark <james.clark@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Leo Yan <leo.yan@...aro.org>
Subject: [RFCv1 2/4] arm64: entry: Always apply workaround for contextidr_el1

After introducing static key as the switch for tracing PID into
contextidr_el1, the kernel can dynamically turn on the static key to
use PID tracing to contextidr_el1.  This means even the config
CONFIG_PID_IN_CONTEXTIDR is not selected, the kernel still can use
contextidr_el1.

When erratum 84571 is detected, the workaround should be always
applied on contextidr_el1, particularly if the static key is enabled
dynamically.

This patch would introduce minor overload by one extra instruction for
accessing system register contextidr_el1 and it only impacts platform
which erratum 84571.  So it's expected to not cause any significant
regression.

Signed-off-by: Leo Yan <leo.yan@...aro.org>
---
 arch/arm64/kernel/entry.S | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index bc6d5a970a13..c41a4cfff527 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -356,12 +356,8 @@ alternative_else_nop_endif
 
 #ifdef CONFIG_ARM64_ERRATUM_845719
 alternative_if ARM64_WORKAROUND_845719
-#ifdef CONFIG_PID_IN_CONTEXTIDR
 	mrs	x29, contextidr_el1
 	msr	contextidr_el1, x29
-#else
-	msr contextidr_el1, xzr
-#endif
 alternative_else_nop_endif
 #endif
 3:
-- 
2.25.1