[PATCH 1/1] x86/acpi: acpi_is_processor_usable() dropping possible cpus

[Eric DeVolder <eric.devolder@oracle.com>]

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@oracle.com>
Suggested-by: Boris Ostrovsky <boris.ovstrosky@oracle.com>
Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
---
 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