Re: [PATCH RFC] x86: Reduce MAX_LOCAL_APIC and MAX_IO_APICS

[Denys Vlasenko <dvlasenk@redhat.com>]

On 09/30/2015 05:11 PM, Thomas Gleixner wrote:
> On Fri, 25 Sep 2015, Denys Vlasenko wrote:
>>  
>> +config MAX_LAPIC_ID
>> +	int "Maximum APIC ID"
>> +	range 8 32768
>> +	default "8"
>> +	---help---
>> +	  Use this option to set maximum allowed Local APIC ID higher than
>> +	  maximum number of CPUs. This may be necessary for machines
>> +	  with large number of processor sockets and non-contiguous
>> +	  LAPIC numbering.
>> +	  This setting will be automatically rounded up, if necessary.
> 
> This is wrong. If you would limit the APIC IDs then you really break
> stuff. You can only limit the number of APICs.

This CONFIG setting allows to _increase_ max APIC ID.

Check out this part of the patch:

+/*
+ * Allow non-contiguous APIC IDs for small machines:
+ * APIC ids 0..15 are valid in any config.
+ * Typical SMP machines have contiguous APIC IDs: 0..NR_CPUS-1.
+ * CONFIG_MAX_LAPIC_ID can override.
+ */
+#define MAX_LOCAL_APIC (NR_CPUS < 16 ? 16 : NR_CPUS)
+#if MAX_LOCAL_APIC < CONFIG_MAX_LAPIC_ID
+# undef  MAX_LOCAL_APIC
+# define MAX_LOCAL_APIC CONFIG_MAX_LAPIC_ID
 #endif


For example, if you'd build with NR_CPUS=128
(for example, Fedora kernels do that),
max accepted APIC id will be NR_CPUS-1 = 127
even if CONFIX_MAX_LAPIC_ID is 8.

If Fedora would want to support APIC ids up to
255, it will need to set CONFIG_MAX_LAPIC_ID=256.

Otherwise, if it's happy with "only" supporting up to 128,
it does not need to change CONFIG_MAX_LAPIC_ID from default.

With current kernels, max APIC id for any kernel is 32768,
which is in most cases way bigger than necessary.


Perhaps I need to update the text.
Something like:

- This setting will be automatically rounded up, if necessary
+ This setting will be increased to NR_CPUS, if necessary


> ACPI: LAPIC (acpi_id[0x00] lapic_id[0x00] enabled)
> ACPI: LAPIC (acpi_id[0x01] lapic_id[0x02] enabled)

Does it mean that on a 2-CPU machine, CPU #1 has APIC_ID=2?

My patch will work fine for this machine,
with any CONFIG_MAX_LAPIC_ID.