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: Will Deacon <will@xxxxxxxxxx> --- arch/arm64/kernel/cpufeature.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c index 29017cbb6c8e..fe470683b43e 100644 --- a/arch/arm64/kernel/cpufeature.c +++ b/arch/arm64/kernel/cpufeature.c @@ -1237,6 +1237,8 @@ has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope) 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); @@ -1245,6 +1247,22 @@ static int enable_mismatched_32bit_el0(unsigned int cpu) static_branch_enable_cpuslocked(&arm64_mismatched_32bit_el0); } + 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; + pr_info("Asymmetric 32-bit EL0 support detected on CPU %u; CPU hot-unplug disabled on CPU %u\n", + cpu, lucky_winner); return 0; } -- 2.29.2.454.gaff20da3a2-goog