PERFORCE change 117140 for review

[Marcel Moolenaar]

On Apr 2, 2007, at 8:55 AM, John Baldwin wrote:

>> For example, to analyze machine checks on on pluto1, I disabled CPU0
>> and CPU1 in succession to see if one of the CPUs was the cause of the
>> MC. As such, CPU1 had to be the BSP when CPU0 was disabled. Luckily
>> pluto1 is only a dual-CPU machine, so that disabling a CPU also stops
>> SMP operation :-)
>
> FreeBSD CPU ID's != firmware CPU IDs.

It is important, and not only for identification, that the logical  
CPU ID
used by FreeBSD is the same as used by the firmware (if at all  
possible).
A different ID only causes confusion, especially when the firmware draws
its IDs from the same domain. What is called CPU4 within FreeBSD may not
be called CPU4 in the firmware, even though CPU4 may exist.

>   CPU ID's != local APIC ID's on x86 for example, nor
> are they identical to the CPU indices in the hwprb on Alpha.

They are also not the same on ia64. In fact, on ia64 the APIC ID
consists of 2 elements. The ACPI ID is exactly the kind of ID we
want...

>   FreeBSD CPU
> ID's tend to not be sparse for example, because they are completely  
> separate
> from firmware IDs.

This statement is flawed. Sparseness is unavoidable when CPUs can be
hot-plugged. While we should have a CPU ID that maps trivially to a
bit field for masking purposes, there's no reason to allow sparse IDs
to certain extend.

Both ACPI and Open Firmware have CPU IDs that map trivially to a mask
and they tend to be dense. I see no reason to not use the firmware IDs
as FreeBSD's notion of CPU ID. In fact, I see reasons not to create
our own IDs. In such reason is the added overhead of mapping from one
to the other during runtime.

I think that testing for CPU0 when we really want to know if we're
running on the BSP is also flawed. On ia64 there's typically 1 BSP
that is used to boot the machine, but each cell in a NUMA system
has a monarch CPU that serves the purpose of the BSP for that cell.
This means that some tests that check for the BSP may need to be
changed to check for the monarch instead. Since there can obviously
be only 1 CPU0, there will (ipso facto) be BSP-like processors with
an ID != 0. It's therefore better not to assume "special powers" for
CPU0 and instead check the PCPU for flags that corresponds 1-on-1
with such powers.

-- 
Marcel Moolenaar
xcllnt at mac.com