Re: [PATCH RFC v4 11/15] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs

On Wed, 31 Jan 2024 16:50:32 +0000
Russell King (Oracle) <[email protected]> wrote:

> From: James Morse <[email protected]>
> 
> 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 <[email protected]>
> Signed-off-by: Russell King (Oracle) <[email protected]>

Reviewed-by: Jonathan Cameron <[email protected]>