arm64: Exclude nohz_full CPUs from 32bits el0 support

Nohz full CPUs are not a desirable fallback target to run 32bits el0
applications. If present, prefer a set of housekeeping CPUs that can do
the job instead. Otherwise just don't support el0 32 bits. Should the
need arise, appropriate support can be introduced in the future.

Suggested-by: Will Deacon <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

Author: Frederic Weisbecker

Documentation/arch/arm64/asymmetric-32bit.rst

@@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
 mode will return to host userspace with an ``exit_reason`` of
 ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
 re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
+
+NOHZ FULL
+---------
+
+To avoid perturbing an adaptive-ticks CPU (specified using
+``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
+are treated as 64-bit-only when support for asymmetric 32-bit systems
+is enabled.

arch/arm64/kernel/cpufeature.c

@@ -75,6 +75,7 @@
 #include <linux/cpu.h>
 #include <linux/kasan.h>
 #include <linux/percpu.h>
+#include <linux/sched/isolation.h>
 
 #include <asm/cpu.h>
 #include <asm/cpufeature.h>
@@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
 	static int lucky_winner = -1;
 
 	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
-	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
+	bool cpu_32bit = false;
+
+	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
+		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
+			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
+		else
+			cpu_32bit = true;
+	}
 
 	if (cpu_32bit) {
 		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);