[PATCH] arm64: errata: Fix handling of 1418040 with late CPU onlining

[Will Deacon]

In a surprising turn of events, it transpires that CPU capabilities
configured as ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE are never set as the
result of late-onlining. Therefore our handling of erratum 1418040 does
not get activated if it is not required by any of the boot CPUs, even
though we allow late-onlining of an affected CPU.

In order to get things working again, replace the cpus_have_const_cap()
invocation with an explicit check for the current CPU using
this_cpu_has_cap().

Cc: Marc Zyngier <maz at kernel.org>
Cc: Sai Prakash Ranjan <saiprakash.ranjan at codeaurora.org>
Cc: Stephen Boyd <swboyd at chromium.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: Suzuki Poulose <suzuki.poulose at arm.com>
Signed-off-by: Will Deacon <will at kernel.org>
---

Found by code inspection and compile-tested only, so I would really
appreciate a second look.

 arch/arm64/include/asm/cpufeature.h | 2 ++
 arch/arm64/kernel/process.c         | 5 ++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index f7e7144af174..c59c16a6ea8b 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -268,6 +268,8 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
 /*
  * CPU feature detected at boot time based on feature of one or more CPUs.
  * All possible conflicts for a late CPU are ignored.
+ * NOTE: this means that a late CPU with the feature will *not* cause the
+ * capability to be advertised by cpus_have_*cap()!
  */
 #define ARM64_CPUCAP_WEAK_LOCAL_CPU_FEATURE		\
 	(ARM64_CPUCAP_SCOPE_LOCAL_CPU		|	\
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 4784011cecac..a47a40ec6ad9 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -522,14 +522,13 @@ static void erratum_1418040_thread_switch(struct task_struct *prev,
 	bool prev32, next32;
 	u64 val;
 
-	if (!(IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) &&
-	      cpus_have_const_cap(ARM64_WORKAROUND_1418040)))
+	if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040))
 		return;
 
 	prev32 = is_compat_thread(task_thread_info(prev));
 	next32 = is_compat_thread(task_thread_info(next));
 
-	if (prev32 == next32)
+	if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
 		return;
 
 	val = read_sysreg(cntkctl_el1);
-- 
2.29.1.341.ge80a0c044ae-goog