Messages in this thread | | | Date | Fri, 04 Jul 2003 13:37:56 -0700 | From | "Martin J. Bligh" <> | Subject | Re: 2.5.74-mm1 fails to boot due to APIC trouble, 2.5.73mm3 works. |
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>> Maybe not, but it looks like one. Maybe if you actually explain >> what you're trying to fix, and why? >> I think this kind of change deserves a better explanation that >> "I'm right" ... that's my main objection. > > I'll try to be more verbose, then.
Thanks ... will help a lot ;-)
> On Fri, Jul 04, 2003 at 12:38:19PM -0700, Martin J. Bligh wrote: >> Explain. Not obvious to the casual observer. > > The function assigns physical APIC ID's to IO-APIC's. The loop is > intended to iterate over the physical APIC ID space. 0xf is an > inaccurate description of the upper bound on the physical APIC ID space. > APIC_BROADCAST_ID is a more accurate upper bound.
OK, you're right. Is just confusing because it works as it is right now ... even on a 32x system - however, that's only because Summit doesn't actually run that region of code, and NUMA-Q ignores it.
> On Fri, Jul 04, 2003 at 02:35:31AM -0700, William Lee Irwin III wrote: >>> APIC_BROADCAST_ID is an upper bound on valid physical APIC ID's as it >>> is used in the code. That actually was commented in the patch. > > On Fri, Jul 04, 2003 at 12:38:19PM -0700, Martin J. Bligh wrote: >> I find it odd that this worked before then. Also seems to be a separate >> issue from the rest of the patch. Is quite probably correct, is just >> non-obvious in the context of the rest of the patch. > > I audited not only for usage of limited-width bitmaps for APIC ID > spaces, but also improper bounds on iterations over APIC ID spaces. > Things ran out of APIC ID's when phys_cpu_present_map was NR_CPUS > wide. This patch makes the limits accurate to the hardware with > the brute-force application of bitmaps. The semantic impact of > dropping in a bitmap is very low. The issue that arose was that it > wasn't wide enough, which was obvious enough to spot as a thinko > without even testing. > >> Why is Summit 0xF, and bigsmp 0xFF then? > > Summit (and all other xAPIC-based subarches) should be 0xFF; I missed > it in the sweep.
OK. Makes more sense now if both are that way.
> On Fri, Jul 04, 2003 at 02:35:31AM -0700, William Lee Irwin III wrote: >>> Look at where it's used. > > On Fri, Jul 04, 2003 at 12:38:19PM -0700, Martin J. Bligh wrote: >> I did. Still unclear why you think this is correct, or what physical >> apicids have to do with a function that maps from apicids to the >> phys_cpu_present_map, which is a compact mapping of logical apicids >> for NUMA-Q. >> Sorry, but this needs more explanation. > > The bitmap width is sufficient. NUMA-Q abuses what everything else > uses for physical APIC ID's (partly because of the BIOS). It so happens > that the array is MAX_APICS wide, which suffices for NUMA-Q (and > anything else that cares to use it). > > No. This was not written for or around NUMA-Q; it's meant for the > io_apic.c loops and sparse physid wakeup on non-NUMA-Q machines.
OK, maybe this is just an extension of my earlier abuse - in which case, let's just remove it. Was bad enough before, but now even I can't understand it, and I wrote the damned thing.
So yes, it looks correct. I'll see if I can see a neat way to bury this under the existing abstractions like Summit does ... I'm not sure it's a good idea to have two different methods for this; that whole area of code is getting horribly complicated ...
Thanks very much for the explanations,
M.
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