[PATCH v8 11/16] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs

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From: James Morse <james.morse@xxxxxxx>

To support virtual CPU hotplug, ACPI has added an 'online capable' bit
to the MADT GICC entries. This indicates a disabled CPU entry may not
be possible to online via PSCI until firmware has set enabled bit in

This means that a "usable" GIC is one that is marked as either enabled,
or online capable. Therefore, change acpi_gicc_is_usable() to check both
bits. However, we need to change the test in gic_acpi_match_gicc() back
to testing just the enabled bit so the count of enabled distributors is

What about the redistributor in the GICC entry? ACPI doesn't want to say.
Assume the worst: When a redistributor is described in the GICC entry,
but the entry is marked as disabled at boot, assume the redistributor
is inaccessible.

The GICv3 driver doesn't support late online of redistributors, so this
means the corresponding CPU can't be brought online either.
Rather than modifying cpu masks that may already have been used,
register a new cpuhp callback to fail this case. This must run earlier
than the main gic_starting_cpu() so that this case can be rejected
before the section of cpuhp that runs on the CPU that is coming up as
that is not allowed to fail. This solution keeps the handling of this
broken firmware corner case local to the GIC driver. As precise ordering
of this callback doesn't need to be controlled as long as it is
in that initial prepare phase, use CPUHP_BP_PREPARE_DYN.

Systems that want CPU hotplug in a VM can ensure their redistributors
are always-on, and describe them that way with a GICR entry in the MADT.

Suggested-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: James Morse <james.morse@xxxxxxx>
Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
Tested-by: Miguel Luis <miguel.luis@xxxxxxxxxx>
Co-developed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>

Thanks to Marc for review and suggestions!
v8: Change the handling of broken rdists to fail cpuhp rather than
    modifying the cpu_present and cpu_possible masks.
    Updated commit text to reflect that.
    Added a sb tag for Marc given this is more or less what he put
    in his review comment.
 drivers/irqchip/irq-gic-v3.c | 38 ++++++++++++++++++++++++++++++++++--
 include/linux/acpi.h         |  3 ++-
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
index 10af15f93d4d..b4685991953e 100644
--- a/drivers/irqchip/irq-gic-v3.c
+++ b/drivers/irqchip/irq-gic-v3.c
@@ -44,6 +44,8 @@
+static struct cpumask broken_rdists __read_mostly;
 struct redist_region {
 	void __iomem		*redist_base;
 	phys_addr_t		phys_base;
@@ -1293,6 +1295,18 @@ static void gic_cpu_init(void)
 #define MPIDR_TO_SGI_RS(mpidr)	(MPIDR_RS(mpidr) << ICC_SGI1R_RS_SHIFT)
 #define MPIDR_TO_SGI_CLUSTER_ID(mpidr)	((mpidr) & ~0xFUL)
+ * gic_starting_cpu() is called after the last point where cpuhp is allowed
+ * to fail. So pre check for problems earlier.
+ */
+static int gic_check_rdist(unsigned int cpu)
+	if (cpumask_test_cpu(cpu, &broken_rdists))
+		return -EINVAL;
+	return 0;
 static int gic_starting_cpu(unsigned int cpu)
@@ -1384,6 +1398,10 @@ static void __init gic_smp_init(void)
 	int base_sgi;
+	cpuhp_setup_state_nocalls(CPUHP_BP_PREPARE_DYN,
+				  "irqchip/arm/gicv3:checkrdist",
+				  gic_check_rdist, NULL);
 				  gic_starting_cpu, NULL);
@@ -2363,11 +2381,24 @@ gic_acpi_parse_madt_gicc(union acpi_subtable_headers *header,
 				(struct acpi_madt_generic_interrupt *)header;
 	u32 reg = readl_relaxed(acpi_data.dist_base + GICD_PIDR2) & GIC_PIDR2_ARCH_MASK;
 	u32 size = reg == GIC_PIDR2_ARCH_GICv4 ? SZ_64K * 4 : SZ_64K * 2;
+	int cpu = get_cpu_for_acpi_id(gicc->uid);
 	void __iomem *redist_base;
 	if (!acpi_gicc_is_usable(gicc))
 		return 0;
+	/*
+	 * Capable but disabled CPUs can be brought online later. What about
+	 * the redistributor? ACPI doesn't want to say!
+	 * Virtual hotplug systems can use the MADT's "always-on" GICR entries.
+	 * Otherwise, prevent such CPUs from being brought online.
+	 */
+	if (!(gicc->flags & ACPI_MADT_ENABLED)) {
+		pr_warn_once("CPU %u's redistributor is inaccessible: this CPU can't be brought online\n", cpu);
+		cpumask_set_cpu(cpu, &broken_rdists);
+		return 0;
+	}
 	redist_base = ioremap(gicc->gicr_base_address, size);
 	if (!redist_base)
 		return -ENOMEM;
@@ -2413,9 +2444,12 @@ static int __init gic_acpi_match_gicc(union acpi_subtable_headers *header,
 	 * If GICC is enabled and has valid gicr base address, then it means
-	 * GICR base is presented via GICC
+	 * GICR base is presented via GICC. The redistributor is only known to
+	 * be accessible if the GICC is marked as enabled. If this bit is not
+	 * set, we'd need to add the redistributor at runtime, which isn't
+	 * supported.
-	if (acpi_gicc_is_usable(gicc) && gicc->gicr_base_address)
+	if (gicc->flags & ACPI_MADT_ENABLED && gicc->gicr_base_address)
 	return 0;
diff --git a/include/linux/acpi.h b/include/linux/acpi.h
index 9844a3f9c4e5..fcfb7bb6789e 100644
--- a/include/linux/acpi.h
+++ b/include/linux/acpi.h
@@ -239,7 +239,8 @@ void acpi_table_print_madt_entry (struct acpi_subtable_header *madt);
 static inline bool acpi_gicc_is_usable(struct acpi_madt_generic_interrupt *gicc)
-	return gicc->flags & ACPI_MADT_ENABLED;
+	return gicc->flags & (ACPI_MADT_ENABLED |
 /* the following numa functions are architecture-dependent */

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