ACPI 6.3 introduced a flag allowing to tell MADT entries describing
hotpluggable processors from ones which are simply placeholders (often
used by firmware writers to simplify handling there).

Inspired by a Linux patch by Mario Limonciello <mario.limonciello@amd.com>.

Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/acpi/boot.c
+++ b/xen/arch/x86/acpi/boot.c
@@ -53,6 +53,8 @@ bool __initdata acpi_ioapic;
 static bool __initdata acpi_skip_timer_override;
 boolean_param("acpi_skip_timer_override", acpi_skip_timer_override);
 
+static uint8_t __initdata madt_revision;
+
 static u64 acpi_lapic_addr __initdata = APIC_DEFAULT_PHYS_BASE;
 
 /* --------------------------------------------------------------------------
@@ -64,6 +66,8 @@ static int __init acpi_parse_madt(struct
 	struct acpi_table_madt *madt =
 		container_of(table, struct acpi_table_madt, header);
 
+	madt_revision = madt->header.revision;
+
 	if (madt->address) {
 		acpi_lapic_addr = (u64) madt->address;
 
@@ -86,6 +90,12 @@ acpi_parse_x2apic(struct acpi_subtable_h
 	if (BAD_MADT_ENTRY(processor, end))
 		return -EINVAL;
 
+	/* Don't register processors that cannot be onlined. */
+	if (madt_revision >= 5 &&
+	    !(processor->lapic_flags & ACPI_MADT_ENABLED) &&
+	    !(processor->lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+		return 0;
+
 	if ((processor->lapic_flags & ACPI_MADT_ENABLED) ||
 	    processor->local_apic_id != 0xffffffff || opt_cpu_info) {
 		acpi_table_print_madt_entry(header);
@@ -136,6 +146,12 @@ acpi_parse_lapic(struct acpi_subtable_he
 	if (BAD_MADT_ENTRY(processor, end))
 		return -EINVAL;
 
+	/* Don't register processors that cannot be onlined. */
+	if (madt_revision >= 5 &&
+	    !(processor->lapic_flags & ACPI_MADT_ENABLED) &&
+	    !(processor->lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+		return 0;
+
 	if ((processor->lapic_flags & ACPI_MADT_ENABLED) ||
 	    processor->id != 0xff || opt_cpu_info)
 		acpi_table_print_madt_entry(header);
--- a/xen/include/acpi/actbl1.h
+++ b/xen/include/acpi/actbl1.h
@@ -858,7 +858,8 @@ struct acpi_madt_generic_translator {
 
 /* MADT Local APIC flags */
 
-#define ACPI_MADT_ENABLED           (1)	/* 00: Processor is usable if set */
+#define ACPI_MADT_ENABLED           (1 << 0)	/* 00: Processor is usable if set */
+#define ACPI_MADT_ONLINE_CAPABLE    (1 << 1)	/* 01: Processor can be onlined */
 
 /* MADT MPS INTI flags (inti_flags) */
 

On Mon, Sep 13, 2021 at 09:54:01AM +0200, Jan Beulich wrote:
> ACPI 6.3 introduced a flag allowing to tell MADT entries describing
> hotpluggable processors from ones which are simply placeholders (often
> used by firmware writers to simplify handling there).
> 
> Inspired by a Linux patch by Mario Limonciello <mario.limonciello@amd.com>.
> 
> Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.