Commit df950811 authored by Will Deacon's avatar Will Deacon Committed by Peter Zijlstra
Browse files

arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system 



If we want to support 32-bit applications, then when we identify a CPU
with mismatched 32-bit EL0 support we must ensure that we will always
have an active 32-bit CPU available to us from then on. This is important
for the scheduler, because is_cpu_allowed() will be constrained to 32-bit
CPUs for compat tasks and forced migration due to a hotplug event will
hang if no 32-bit CPUs are available.

On detecting a mismatch, prevent offlining of either the mismatching CPU
if it is 32-bit capable, or find the first active 32-bit capable CPU
otherwise.

Signed-off-by: default avatarWill Deacon <will@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210730112443.23245-13-will@kernel.org
parent 08cd8f41

arch/arm64/kernel/cpufeature.c

+24 −1
Original line number Diff line number Diff line
@@ -2901,15 +2901,38 @@ void __init setup_cpu_features(void) 


static int enable_mismatched_32bit_el0(unsigned int cpu)

{

	/*

	 * The first 32-bit-capable CPU we detected and so can no longer

	 * be offlined by userspace. -1 indicates we haven't yet onlined

	 * a 32-bit-capable 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);



	if (cpu_32bit) {

		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);

		static_branch_enable_cpuslocked(&arm64_mismatched_32bit_el0);

		setup_elf_hwcaps(compat_elf_hwcaps);

	}



	if (cpumask_test_cpu(0, cpu_32bit_el0_mask) == cpu_32bit)

		return 0;



	if (lucky_winner >= 0)

		return 0;



	/*

	 * We've detected a mismatch. We need to keep one of our CPUs with

	 * 32-bit EL0 online so that is_cpu_allowed() doesn't end up rejecting

	 * every CPU in the system for a 32-bit task.

	 */

	lucky_winner = cpu_32bit ? cpu : cpumask_any_and(cpu_32bit_el0_mask,

							 cpu_active_mask);

	get_cpu_device(lucky_winner)->offline_disabled = true;

	setup_elf_hwcaps(compat_elf_hwcaps);

	pr_info("Asymmetric 32-bit EL0 support detected on CPU %u; CPU hot-unplug disabled on CPU %u\n",

		cpu, lucky_winner);

	return 0;

}