Random x86-64 seg-fault finally fixed

[Matthew Dillon]

:Just out of curiosity, are there any plans for supporting
:machines with 64 cores or even more?
:
:For example, the Sun Fire X4800 (quite common in larger data
:centers) supports eight Xeon 7500 packages which have eight
:cores plus hyperthreading, which gives a 128-way SMP system.
:Solaris, Linux and Windows support them, but I don't think
:any of the BSDs does.
:
:And that's certainly not the end.  256-way x86 systems have
:been announced already.  Basically, it seems that the growth
:of the number of cores has taken over the role of the growth
:of clockrate (because the latter starts to face physical
:limits).
:
:Best regards
:   Oliver

    The basic limitation here for us the convenience of being able to
    operate atomically on cpu masks and in particular being able to
    run bit-search instructions on them (bsfq, etc).  There are also
    a few places where I combine cpu masking operations with a spin
    lock, which is ultra convenient.  I would have to rewrite those
    algorithms to operate on an array of cpu masks instead of just one.
    It isn't in the cards right now though if someone threw a 128-way
    box onto my carpet I'd probably start working on it :-)

    Taking x86-64 to 63 cpus was easy because we have 64-bit bit
    instructions.  I'm stealing a bit in the cpumask for a pmap
    spinlock, or it would be 64.

    Beyond that it probably makes sense to consider partitioning the
    hardware with or without virtualization.  Partitioning is already
    desireable for the current 48-core monster and I'd like to have
    some sort of DragonFly host & guest solution that runs at full
    performance on the bare HW without virtualization.

						-Matt