drivers/perf: arm_pmu_acpi: avoid perf IRQ init when guest PMU is off

We saw perf IRQ init failures when running Linux kernel in an ACPI
guest without PMU (i.e. pmu=off). This is because perf IRQ is not
present when pmu=off, but arm_pmu_acpi still tries to register
or unregister GSI. This patch addresses the problem by checking
gicc->performance_interrupt. If it is 0, which is the value set
by qemu when pmu=off, we skip the IRQ register/unregister process.

[    4.069470] bc00: 0000000000040b00 ffff0000089db190
[    4.070267] [<ffff000008134f80>] enable_percpu_irq+0xdc/0xe4
[    4.071192] [<ffff000008667cc4>] arm_perf_starting_cpu+0x108/0x10c
[    4.072200] [<ffff0000080cbdd4>] cpuhp_invoke_callback+0x14c/0x4ac
[    4.073210] [<ffff0000080ccd3c>] cpuhp_thread_fun+0xd4/0x11c
[    4.074132] [<ffff0000080f1394>] smpboot_thread_fn+0x1b4/0x1c4
[    4.075081] [<ffff0000080ec90c>] kthread+0x10c/0x138
[    4.075921] [<ffff0000080833c0>] ret_from_fork+0x10/0x50
[    4.076947] genirq: Setting trigger mode 4 for irq 43 failed
(gic_set_type+0x0/0x74)

Signed-off-by: Wei Huang <wei@redhat.com>
[will: add comment justifying deviation from ACPI spec, removed redundant hunk]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

drivers/perf/arm_pmu_acpi.c

@@ -29,6 +29,17 @@ static int arm_pmu_acpi_register_irq(int cpu)
 		return -EINVAL;
 
 	gsi = gicc->performance_interrupt;
+
+	/*
+	 * Per the ACPI spec, the MADT cannot describe a PMU that doesn't
+	 * have an interrupt. QEMU advertises this by using a GSI of zero,
+	 * which is not known to be valid on any hardware despite being
+	 * valid per the spec. Take the pragmatic approach and reject a
+	 * GSI of zero for now.
+	 */
+	if (!gsi)
+		return 0;
+
 	if (gicc->flags & ACPI_MADT_PERFORMANCE_IRQ_MODE)
 		trigger = ACPI_EDGE_SENSITIVE;
 	else