[PATCH v7 19/22] arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system

Will Deacon
2021-05-25T15:36:23Z

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.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
arch/arm64/kernel/cpufeature.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 959442f76ed7..72efdc611b14 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2896,15 +2896,33 @@ void __init setup_cpu_features(void)

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);

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;
}

--
2.31.1.818.g46aad6cb9e-goog