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

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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@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210730112443.23245-13-will@kernel.org
This commit is contained in:
Will Deacon 2021-07-30 12:24:39 +01:00 committed by Peter Zijlstra
parent 08cd8f4112
commit df950811f4
1 changed files with 24 additions and 1 deletions
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25
arch/arm64/kernel/cpufeature.c
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@ -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;
}