Re: [PATCH 8/8] x86_64: Support for new UV apic

Jack Steiner
2008-03-28T20:24:42Z

On Fri, Mar 28, 2008 at 09:15:32PM +0100, Ingo Molnar wrote:
>
> * Jack Steiner <steiner@sgi.com> wrote:
>
> > Index: linux/arch/x86/kernel/apic_64.c
> > ===================================================================
> > --- linux.orig/arch/x86/kernel/apic_64.c 2008-03-28 13:00:22.000000000 -0500
> > +++ linux/arch/x86/kernel/apic_64.c 2008-03-28 13:06:12.000000000 -0500
> > @@ -738,6 +738,7 @@ void __cpuinit setup_local_APIC(void)
> > unsigned int value;
> > int i, j;
> >
> > + preempt_disable();
> > value = apic_read(APIC_LVR);
> >
> > BUILD_BUG_ON((SPURIOUS_APIC_VECTOR & 0x0f) != 0x0f);
> > @@ -831,6 +832,7 @@ void __cpuinit setup_local_APIC(void)
> > else
> > value = APIC_DM_NMI | APIC_LVT_MASKED;
> > apic_write(APIC_LVT1, value);
> > + preempt_enable();
> > }
>
> hm, this looks a bit weird - why are all the preempt-disable/enable
> calls needed?

The first patch had a preempt disable/enable in the function
that reads apicid (see read_apic_id() in arch/x86/kernel/genapic_64.c).
This seemed necessary since large system generate an apicid by reading
the live id & concatenating it with extra bits.

One of the review comments suggested that I change the preempt to a WARN()
since reading apic_id really should be done with preemtion disabled. The
added code eliminates the warnings.

--- jack