On 4/3/2024 11:12 PM, Igor Mammedov wrote:
On Wed, 3 Apr 2024 10:59:53 -0400
Xiaoyao Li <xiaoyao...@intel.com> wrote:
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. Otherwise, the guest thinks there is a present PIC.
Can you add to commit message reproducer (aka qemu CLI and relevant
logs/symptoms observed on guest side)?
When booting a VM with "-machine xx,pic=off", there is supposed to be no
PIC for the guest. When guest probes PIC, it should find nothing and log
of below should be printed:
[ 0.155970] Using NULL legacy PIC
However, the fact is that no such log printed in guest kernel, with the
VM created with "pic=off". This is because guest think there is a
present due to pcat_compat is reporte as 1 in MADT. See Linux code of
probe_8259A() in arch/x86/kernel/i8259.c
Signed-off-by: Xiaoyao Li <xiaoyao...@intel.com>
---
changes in v2:
- Clarify more in commit message;
---
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 20f19269da40..0cc2919bb851 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);
