Re: [Qemu-devel] [PATCH 6/9] pc: acpi: create MADT.lapic entries only for valid lapics

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 5 Feb 2016 13:28:31 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Feb 04, 2016 at 12:47:32PM +0100, Igor Mammedov wrote:
> > do not assume that all lapics in range 0..apic_id_limit
> > are valid and do not create lapic entries for not
> > possible lapics in MADT.
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>  
> 
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
> 
> But there's one minor suggestion below:
> 
> > ---
> >  hw/i386/acpi-build.c | 21 ++++++++++++++-------
> >  1 file changed, 14 insertions(+), 7 deletions(-)
> > 
> > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> > index df13c7d..9eeeffa 100644
> > --- a/hw/i386/acpi-build.c
> > +++ b/hw/i386/acpi-build.c
> > @@ -361,9 +361,11 @@ build_fadt(GArray *table_data, GArray *linker, AcpiPmInfo *pm,
> >  }
> >  
> >  static void
> > -build_madt(GArray *table_data, GArray *linker, AcpiCpuInfo *cpu,
> > -           PcGuestInfo *guest_info)
> > +build_madt(GArray *table_data, GArray *linker,
> > +           MachineState *machine, PcGuestInfo *guest_info)
> >  {
> > +    MachineClass *mc = MACHINE_GET_CLASS(machine);
> > +    GArray *apic_id_list = mc->possible_cpu_arch_ids();
> >      int madt_start = table_data->len;
> >  
> >      AcpiMultipleApicTable *madt;
> > @@ -376,18 +378,23 @@ build_madt(GArray *table_data, GArray *linker, AcpiCpuInfo *cpu,
> >      madt->local_apic_address = cpu_to_le32(APIC_DEFAULT_ADDRESS);
> >      madt->flags = cpu_to_le32(1);
> >  
> > -    for (i = 0; i < guest_info->apic_id_limit; i++) {
> > +    for (i = 0; i < apic_id_list->len; i++) {
> >          AcpiMadtProcessorApic *apic = acpi_data_push(table_data, sizeof *apic);
> > +        CPUArchId id = FETCH_CPU_ARCH_ID(apic_id_list, i);
> > +        int apic_id = id.arch_id;
> > +
> >          apic->type = ACPI_APIC_PROCESSOR;
> >          apic->length = sizeof(*apic);
> > -        apic->processor_id = i;
> > -        apic->local_apic_id = i;
> > -        if (test_bit(i, cpu->found_cpus)) {
> > +        apic->processor_id = apic_id;
> > +        apic->local_apic_id = apic_id;
> > +        if (id.cpu != NULL) {  
> 
> This seems to be the only place where CPUArchId.cpu is being used
> (see my previous suggestion about making possible_cpu_arch_ids()
> return just an uint64_t list).
> 
> Also, using the existing found_cpus bitmap is more efficient than
> making multiple calls to qemu_get_cpu_by_arch_id(). I wouldn't
> mind keeping the bitmap.
found_cpus bitmap is not better than (id.cpu != NULL) check
the cost of filling both is about the same.
The issue I have with bitmap is that it's harder to generalize
CPU hotplug code with it, while with possible_cpu_arch_ids()
returned array I have a list of CPUs to work with without any
assumptions on position in bitmap or array.
Also bitmap scales worse than a list of CPUs if ID space
is sparse and if ID is quite big.
That's why I'm dropping bitmap and switching to a list of
IDs which in worst case is upto max_cpus.

> 
> >              apic->flags = cpu_to_le32(1);
> >          } else {
> >              apic->flags = cpu_to_le32(0);
> >          }
> >      }
> > +    g_array_free(apic_id_list, true);
> > +
> >      io_apic = acpi_data_push(table_data, sizeof *io_apic);
> >      io_apic->type = ACPI_APIC_IO;
> >      io_apic->length = sizeof(*io_apic);
> > @@ -2659,7 +2666,7 @@ void acpi_build(PcGuestInfo *guest_info, AcpiBuildTables *tables)
> >      aml_len += tables_blob->len - fadt;
> >  
> >      acpi_add_table(table_offsets, tables_blob);
> > -    build_madt(tables_blob, tables->linker, &cpu, guest_info);
> > +    build_madt(tables_blob, tables->linker, machine, guest_info);
> >  
> >      if (misc.has_hpet) {
> >          acpi_add_table(table_offsets, tables_blob);
> > -- 
> > 1.8.3.1
> >   
>