arm64: dump cpu_hwcaps at panic time

When debugging a kernel panic(), it can be useful to know which CPU
features have been detected by the kernel, as some code paths can depend
on these (and may have been patched at runtime).

This patch adds a notifier to dump the detected CPU caps (as a hex
string) at panic(), when we log other information useful for debugging.
On a Juno R1 system running v4.12-rc5, this looks like:

[  615.431249] Kernel panic - not syncing: Fatal exception in interrupt
[  615.437609] SMP: stopping secondary CPUs
[  615.441872] Kernel Offset: disabled
[  615.445372] CPU features: 0x02086
[  615.448522] Memory Limit: none

A developer can decode this by looking at the corresponding
<asm/cpucaps.h> bits. For example, the above decodes as:

* bit  1: ARM64_WORKAROUND_DEVICE_LOAD_ACQUIRE
* bit  2: ARM64_WORKAROUND_845719
* bit  7: ARM64_WORKAROUND_834220
* bit 13: ARM64_HAS_32BIT_EL0
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Steve Capper <steve.capper@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/cpufeature.c

@@ -51,6 +51,25 @@ unsigned int compat_elf_hwcap2 __read_mostly;
 DECLARE_BITMAP(cpu_hwcaps, ARM64_NCAPS);
 EXPORT_SYMBOL(cpu_hwcaps);
 
+static int dump_cpu_hwcaps(struct notifier_block *self, unsigned long v, void *p)
+{
+	/* file-wide pr_fmt adds "CPU features: " prefix */
+	pr_emerg("0x%*pb\n", ARM64_NCAPS, &cpu_hwcaps);
+	return 0;
+}
+
+static struct notifier_block cpu_hwcaps_notifier = {
+	.notifier_call = dump_cpu_hwcaps
+};
+
+static int __init register_cpu_hwcaps_dumper(void)
+{
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &cpu_hwcaps_notifier);
+	return 0;
+}
+__initcall(register_cpu_hwcaps_dumper);
+
 DEFINE_STATIC_KEY_ARRAY_FALSE(cpu_hwcap_keys, ARM64_NCAPS);
 EXPORT_SYMBOL(cpu_hwcap_keys);