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: 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

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 0ead8bf..0af5845 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -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;
 }