Re: [PATCH v7 11/16] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs

[Jonathan Cameron]

On Thu, 18 Apr 2024 14:54:07 +0100
Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:

> 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
> _STA.
> 
> 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
> correct.
> 
> 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. Clear the
> possible and present bits.
> 
> 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.
> 
> When mapping redistributors found via GICC entries, handle the case
> where the arch code believes the CPU is present and possible, but it
> does not have an accessible redistributor. Print a warning and clear
> the present and possible bits.
> 
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>

+CC Marc,

Whilst this has been unchanged for a long time, I'm not 100% sure
we've specifically drawn your attention to it before now.

Jonathan

> 
> ---
> v7: No Change.
> ---
>  drivers/irqchip/irq-gic-v3.c | 21 +++++++++++++++++++--
>  include/linux/acpi.h         |  3 ++-
>  2 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/irqchip/irq-gic-v3.c b/drivers/irqchip/irq-gic-v3.c
> index 10af15f93d4d..66132251c1bb 100644
> --- a/drivers/irqchip/irq-gic-v3.c
> +++ b/drivers/irqchip/irq-gic-v3.c
> @@ -2363,11 +2363,25 @@ 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);
> +		set_cpu_present(cpu, false);
> +		set_cpu_possible(cpu, false);
> +		return 0;
> +	}
> +
>  	redist_base = ioremap(gicc->gicr_base_address, size);
>  	if (!redist_base)
>  		return -ENOMEM;
> @@ -2413,9 +2427,12 @@ 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)
>  		acpi_data.enabled_rdists++;
>  
>  	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 |
> +			      ACPI_MADT_GICC_ONLINE_CAPABLE);
>  }
>  
>  /* the following numa functions are architecture-dependent */