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SubjectRe: [PATCH 45/79] [PATCH] fix apic acking of irqs
On Tue, 25 Mar 2008, Glauber Costa wrote:

> the only ESR mention I see in setup_local_APIC() is this:
>
> /* Pound the ESR really hard over the head with a big hammer - mbligh
> */
> if (esr_disable) {
> apic_write(APIC_ESR, 0);
> apic_write(APIC_ESR, 0);
> apic_write(APIC_ESR, 0);
> apic_write(APIC_ESR, 0);
> }
> which seems more like a disablement.

There is more later on...

> the bootup code does clean it, tough, by writing and reading the ESR.

... basically for the original Pentium and Pentium/MMX APIC you only had
to read the ESR to get at the bits. The read would clear them as well as
a side-effect. Although at that stage already it was mentioned in the
spec that for future compatibility a write of zero beforehand (ignored as
the register was r/o) should be performed. Which indeed became a
requirement from PentiumPro onwards as with these processors it was the
write that copied the internal error latches into the visible ESR
register. Except that some Pentium APICs had an erratum, where ESR was
indeed r/w and the leading write of zero would actually clear the register
losing the recorded state, so it had to be avoided despite the
recommendation. Hence the code you can see within:

if (integrated && !esr_disable) {
}
I suppose other APIC implementers were not that keen on keeping bug
compatibility, so chances are other APIC core work just fine as specified
by the architecture (for whatever the meaning of "fine" is).

Note the usual APIC error interrupt handler is smp_error_interrupt().

> > I have asked this question already: what kind of CPU are you running on?
> > Do you really need to have CONFIG_X86_GOOD_APIC clear with it?
> >
> My testings that triggered that were with qemu, with randconfigs. Probably it
> has a good apic, but it is good that it triggered anyway. Otherwise I'd never
> see it.

Ah, I see -- it may be worth checking what actual hardware does and
fixing QEMU if necessary for it to match reality then. ;-)

OTOH, if actual modern hardware triggered such an error, then for the
sake of a generic "runs everywhere" kernel either ack_APIC_irq() or even
apic_write_around() could be modified to perform a run-time check if
configured with !CONFIG_X86_GOOD_APIC and avoid the read if unnecessary;
it's an erratum workaround after all and SMP Pentium systems suffering
from this bug (UP Pentium systems did not nor had a way to enable the
local APIC normally) are probably an insignificant minority if any at all
left these days. Therefore it should be a negligible sacrifice of
performance.
Maciej


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