Re: CFD: XMLification of NOTES

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 04/01/04

  • Next message: Nate Lawson: "Re: newbus ioport usage" 
    To: Bruce Evans <bde@zeta.org.au>
Date: Thu, 1 Apr 2004 11:23:35 -0500

    On Thursday 01 April 2004 12:11 am, Bruce Evans wrote:
    > On Wed, 31 Mar 2004, John Baldwin wrote:
    > > On Tuesday 30 March 2004 06:54 pm, Dag-Erling Smørgrav wrote:
    > > > CPU_I386 should not conflict with SMP, but a kernel build with both
    > > > will be very slow.
    > >
    > > No, it does conflict. There's no cmpxchg on i386 and no one has had the
    > > desire or time to emulate one for 386 machines. Doing so would be a
    > > waste in my opinion as well.
    >
    > des only claimed that it "should not".
    >
    > Emulating cmpxchg might make a kernel built with both slow, but the
    > current CPU_I386 only adds a tiny amount of slowness. It just doesn't
    > work on multi-CPU systems if multiple CPUs are actually used.
    >
    > Does it actually conflict in practice (except for the forced #error)
    > if the hardware is UP? jhb's APIC changes made configuring with SMP
    > not require APIC, so SMP kernels work on UP systems. Configuring with
    > I386_CPU shouldn't affect this, but it does because of the forced #error
    > at compile time.
    >
    > Bruce

    Hmm. Well, if the hardware is UP the kernel still does 'cmpxchg'
    instructions, so you'd still have to emulate cmpxchg for the kernel. There's
    another problem in that I think Peter changed the pmap to make using invltlb
    in place of invlpg be a compile time decision rather than a runtime decision.
    If someone wants to write an in-kernel cmpxchg emulator then I wouldn't mind
    having I386_CPU just turn on the extra runtime checks for invltlb vs. invlpg. 

    -- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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  • Next message: Nate Lawson: "Re: newbus ioport usage"

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