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

  • To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
  • From: "Dave Lively" <dave.lively@xxxxxxxxx>
  • Date: Tue, 17 Oct 2006 09:26:47 -0400
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, "Wang, Winston L" <winston.l.wang@xxxxxxxxx>
  • Delivery-date: Tue, 17 Oct 2006 06:27:22 -0700
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On 10/16/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:

 On 16/10/06 15:51, "Dave Lively" <dave.lively@xxxxxxxxx> wrote:

What about PIC mode support?  OSes expecting just a PIC may
 rely on being able to choose the 4 PCI IRQs.  But I don't know
 this for sure.  In fact, I remember Win2003 with the ACPI PIC HAL
 was successfully working around this problem (by leaving the PCI
 IRQ assignments unchanged), though it complained in the event
 log.  I'm not sure whether we can rely on this or not ...

 I think it's unlikely they'd rely on it. If we don't care about the PIC-mode 
we could keep the PIIX3 emulation

Are you proposing we use the (current) PIIX3 interrupt routing emulation for
PIC mode guests, and simply hide the interrupt routing functionality for APIC
mode?  That would certainly address my concerns about PIC mode (but note
neither of us is convinced my concerns here are legitimate).

(but I'm not sure that most OSes would bother to switch to APIC mode if they 
have only

Mostly I see SMP kernels using it on uniprocessor configs.

one processor). But bothering to try to emulate better PCI-ISA bridge hardware 
feels like
wasted effort if we can sidestep the entire problem.

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.


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