On Mon, May 24, 2021 at 09:32:50PM +0100, Will Deacon wrote:

On Mon, May 24, 2021 at 04:46:58PM +0100, Catalin Marinas wrote:

quoted

On Tue, May 18, 2021 at 10:47:22AM +0100, Will Deacon wrote:

quoted

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;

I don't fully understand this early return. AFAICT, we still call
setup_elf_hwcaps() via setup_cpu_features() if the system supports
32-bit EL0 (mismatched or not) at boot. For CPU hotplug, we can add the
compat hwcaps later if we didn't set them up at boot. So this part is
fine.

However, if CPU0 is 32-bit-capable, it looks like we'd never disable the
offlining on any of the 32-bit-capable CPUs and there's nothing that
prevents offlining CPU0.

That is also deferred until we actually detect the mismatch. For example, if
CPU0 is 32-bit capable but none of the others are, then when we online CPU1
we will print:

  | CPU features: Asymmetric 32-bit EL0 support detected on CPU 1; CPU hot-unplug disabled on CPU 0

so the check above is really asking "Is the CPU being onlined mismatched wrt
the boot CPU?". If yes, then we need to make sure that we're keeping a
32-bit-capable CPU around.

Got it now, the offlining will only be disabled if we detected a
mismatch. For this patch:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

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