The logic in acpi_is_processor_usable() requires the Online Capable
bit be set for hotpluggable cpus. The Online Capable bit is
introduced in ACPI 6.3 and MADT.revision 5.
However, as currently coded, for MADT.revision < 5,
acpi_is_processor_usable() no longer allows for possible hot
pluggable cpus, which is a regressive change in behavior.
This patch restores the behavior where for MADT.revision < 5, the
presence of the lapic/x2apic structure implies a possible hotpluggable
cpu.
Fixes: e2869bd7af60 ("x86/acpi/boot: Do not register processors that cannot be onlined for x2APIC")
Suggested-by: Miguel Luis <miguel.luis@xxxxxxxxxx>
Suggested-by: Boris Ostrovsky <boris.ovstrosky@xxxxxxxxxx>
Signed-off-by: Eric DeVolder <eric.devolder@xxxxxxxxxx>
---
arch/x86/kernel/acpi/boot.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
index 1c38174b5f01..7b5b8ed018b0 100644
--- a/arch/x86/kernel/acpi/boot.c
+++ b/arch/x86/kernel/acpi/boot.c
@@ -193,7 +193,13 @@ static bool __init acpi_is_processor_usable(u32 lapic_flags)
if (lapic_flags & ACPI_MADT_ENABLED)
return true;
- if (acpi_support_online_capable && (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+ /*
+ * Prior to MADT.revision 5, the presence of the Local x2/APIC
+ * structure _implicitly_ noted a possible hotpluggable cpu.
+ * Starting with MADT.revision 5, the Online Capable bit
+ * _explicitly_ indicates a hotpluggable cpu.
+ */
+ if (!acpi_support_online_capable || (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
return true;
return false;
--
2.31.1
