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

[Igor Mammedov]

On Thu, 11 Feb 2016 14:11:34 -0200
Eduardo Habkost <address@hidden> wrote:

> On Fri, Feb 05, 2016 at 05:14:41PM +0100, Igor Mammedov wrote:
> > On Fri, 5 Feb 2016 13:28:31 -0200
> > Eduardo Habkost <address@hidden> 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 <address@hidden>    
> > > 
> > > Reviewed-by: Eduardo Habkost <address@hidden>
> > > 
> > > 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 cost doesn't look the same. Populating found_cpus should be
> O(smp_cpus)[1], and is being done only once. Filling id.cpu in
> pc_possible_cpu_arch_ids() is O(max_cpus*smp_cpus), and it is
> called multiple times.
I've refactored patch to make pc_possible_cpu_arch_ids() linear,
pls see v2:
https://www.mail-archive.com/address@hidden/msg351298.html

> 
> [1] I just noticed it is actually O(size_of_qom_tree), but it
>     is still linear, and could be changed to O(smp_cpus).
> 
> > 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.  
> 
> Yes, I agree that a list is better depending on how the arch ID
> space is used.
> 
> > That's why I'm dropping bitmap and switching to a list of
> > IDs which in worst case is upto max_cpus.  
> 
> I think the possible_cpu_arch_ids() interface looks good, maybe
> we just need to optimize it.
> 
> But I'm not sure if we need to optimize it now, or if we can live
> with the inefficient code and optimize it later. I won't complain
> if we do it later, if we warn about it in the commit message or
> comments.
>