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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
