From: Xiaoyao Li <xiaoyao.li@intel.com>

A value 1 of PCAT_COMPAT (bit 0) of MADT.Flags indicates that the system
also has a PC-AT-compatible dual-8259 setup, i.e., the PIC.  When PIC
is not enabled (pic=off) for x86 machine, the PCAT_COMPAT bit needs to
be cleared.  The PIC probe should then print:

   [    0.155970] Using NULL legacy PIC

However, no such log printed in guest kernel unless PCAT_COMPAT is
cleared.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20240403145953.3082491-1-xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/i386/acpi-common.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/hw/i386/acpi-common.c b/hw/i386/acpi-common.c
index 20f19269da4..0cc2919bb85 100644
--- a/hw/i386/acpi-common.c
+++ b/hw/i386/acpi-common.c
@@ -107,7 +107,9 @@ void acpi_build_madt(GArray *table_data, BIOSLinker *linker,
     acpi_table_begin(&table, table_data);
     /* Local APIC Address */
     build_append_int_noprefix(table_data, APIC_DEFAULT_ADDRESS, 4);
-    build_append_int_noprefix(table_data, 1 /* PCAT_COMPAT */, 4); /* Flags */
+    /* Flags. bit 0: PCAT_COMPAT */
+    build_append_int_noprefix(table_data,
+                              x86ms->pic != ON_OFF_AUTO_OFF ? 1 : 0 , 4);
 
     for (i = 0; i < apic_ids->len; i++) {
         pc_madt_cpu_entry(i, apic_ids, table_data, false);
-- 
2.44.0