Re: [Qemu-devel] [PATCH v3 05/13] pc: leave max apic_id_limit only in legacy cpu hotplug code

[Eduardo Habkost]

On Tue, Oct 18, 2016 at 11:02:54AM +0200, Igor Mammedov wrote:
> On Mon, 17 Oct 2016 19:44:52 -0200
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Thu, Oct 13, 2016 at 11:52:39AM +0200, Igor Mammedov wrote:
> > [...]
> > > @@ -236,7 +237,11 @@ void build_legacy_cpu_hotplug_aml(Aml *ctx, 
> > > MachineState *machine,
> > >      /* The current AML generator can cover the APIC ID range [0..255],
> > >       * inclusive, for VCPU hotplug. */
> > >      QEMU_BUILD_BUG_ON(ACPI_CPU_HOTPLUG_ID_LIMIT > 256);
> > > -    g_assert(pcms->apic_id_limit <= ACPI_CPU_HOTPLUG_ID_LIMIT);
> > > +    if (pcms->apic_id_limit > ACPI_CPU_HOTPLUG_ID_LIMIT) {
> > > +        error_report("max_cpus is too large. APIC ID of last CPU is %u",
> > > +                     pcms->apic_id_limit - 1);
> > > +        exit(1);
> > > +    }  
> > 
> > Moving the check here seems to make sense, but:
> > 
> > >  
> > >      /* create PCI0.PRES device and its _CRS to reserve CPU hotplug MMIO 
> > > */
> > >      dev = aml_device("PCI0." stringify(CPU_HOTPLUG_RESOURCE_DEVICE));
> > > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > > index 93ff49c..f1c1013 100644
> > > --- a/hw/i386/pc.c
> > > +++ b/hw/i386/pc.c
> > > @@ -778,7 +778,6 @@ static FWCfgState *bochs_bios_init(AddressSpace *as, 
> > > PCMachineState *pcms)  
> > 
> > [Added more context below to show the code around the change]
> > 
> > >      numa_fw_cfg = g_new0(uint64_t, 1 + pcms->apic_id_limit + 
> > > nb_numa_nodes);
> > >      numa_fw_cfg[0] = cpu_to_le64(nb_numa_nodes);
> > >      for (i = 0; i < max_cpus; i++) {
> > >          unsigned int apic_id = x86_cpu_apic_id_from_index(i);
> > > -        assert(apic_id < pcms->apic_id_limit);  
> > 
> > If you really needed to remove this assert, that means you can
> > write beyond the end of numa_fw_fg[] below. Are you sure you need
> > to remove it?
> > 
> > >          j = numa_get_node_for_cpu(i);
> > >          if (j < nb_numa_nodes) {
> > >              numa_fw_cfg[apic_id + 1] = cpu_to_le64(j);  
> > 
> >                            ^^^^^^^^^^^ here
> Shouldn't above
>   numa_fw_cfg = g_new0(uint64_t, 1 + pcms->apic_id_limit + nb_numa_nodes);
> allocate sufficiently sized array?

I believe it should, but that would mean the assert() is still
valid.

> 
> That's aside, the assert could be kept as it doesn't get in a way
> if you'd prefer it that way.

Yes, please. The assert() removal seems unnecessary (and
confusing, because it made me believe that the condition was not
going to be valid anymore).

-- 
Eduardo