Re: [PATCH v10 14/19] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs

[Marc Zyngier <maz@kernel.org>]

On Wed, 29 May 2024 14:34:41 +0100,
Jonathan Cameron <Jonathan.Cameron@huawei.com> wrote:
> 
> From: James Morse <james.morse@arm.com>
> 
> 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 redistributor is one that is marked as
> either enabled, or online capable. The meaning of the
> acpi_gicc_is_usable() would become less clear than just checking the
> pair of flags at call sites. As such, drop that helper function.
> The test in gic_acpi_match_gicc() remains as 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.
> 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@kernel.org>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> Tested-by: Miguel Luis <miguel.luis@oracle.com>
> Co-developed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Acked-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Without deviation from the norm, progress is not possible.