arm64: errata: Fix handling of 1418040 with late CPU onlining

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: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Cc: Stephen Boyd <swboyd@chromium.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201106114952.10032-1-will@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

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		|	\

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);