Re: [Xen-devel] [PATCH][HVM] fix PCI interrupt routing via ACPI

["Dave Lively" <dave.lively@xxxxxxxxx>]

On 10/17/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
> On 17/10/06 14:26, "Dave Lively" <dave.lively@xxxxxxxxx> wrote:
> > I'm all for sidestepping the problem when that works.  But I worry
> > that the requirement
> > of supporting PIC mode means we need this general functionality (i.e.,
> > PCI interrupt
> > routing) anyway.  Since there will be no performance advantage in
> > hiding this functionality
> > in APIC mode, perhaps we're better off just leaving it there.
>
> I don't believe that PIC mode requires us to support interrupt routing.
>
>  -- Keir
Sorry, I shouldn't have said "requirement" ...  I agree it's not known
to be necessary (and suspect it's truly not).

Would you agree it's *desirable* for PIC mode guests?  Otherwise they
have to live with the default PCI IRQ assignments, which may lead to
unnecessary IRQ sharing.  But I'm not sure how much of a problem this
is in practice (i.e., since the QEMU configs we use are constrained
anyway).

If we didn't already have a working PCI interrupt router, I wouldn't
be proposing that
we implement one (without further evidence that it helps performance).
But given that
we have one already that works, we might as well use it for PIC mode
guests, at least.

APIC mode guests are a different question.  Once QEMU can raise IRQs >
15, we can either extend the interrupt router to assign IRQs > 15, or
we can hide the interrupt router entirely as you suggest.  I still
suspect the former is easier.  But we don't need to resolve this issue
now.

Dave

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