Re: [PATCH v2] hw/i386/acpi: Set PCAT_COMPAT bit only when pic is not disabled

[Xiaoyao Li]

On 4/3/2024 11:12 PM, Igor Mammedov wrote:
> On Wed,  3 Apr 2024 10:59:53 -0400
> Xiaoyao Li <xiaoyao.li@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.li@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);