linux-kernel - Re: TARGET_CPUS in assign_irq

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Date:	Wed, 10 Sep 2008 13:07:27 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Yinghai Lu <yhlu.kernel@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: TARGET_CPUS in assign_irq_vector

Jeremy Fitzhardinge <jeremy@...p.org> writes:

> Hm.  That would work OK for fully paravirtualized domains, which have no
> direct access to real hardware in any form (well, there's pci
> passthough, but interrupts are all thoroughly massaged into event channels).
>
> But for dom0, the kernel handles interrupts weird hybrid mode.  The
> interrupts themselves are delivered via event channels rather than via a
> local apic, but the IO APIC is still under the kernel's control, and is
> responsible for poking (Xen-allocated) vectors into it.  This only
> applies to physical irq event channels; there's no need to have vectors
> for purely software event channels like interdomain, IPI and timers. 
> This is further complicated by the fact that the dom0 kernel parses the
> ACPI and MPTABLES to find out about IO APICs, so the existing APIC
> subsystem is already involved.  I need to work out how'd I'd hook all
> this together with a minimum of mess.

In that case.  Having the information on the event channel tell you
which cpu and which vector were received is sufficient.  Then you
can call into do_IRQ() with the information.  Unless ack_irq()
and friends are enough different at the local apic level to cause
a challenge.

For the reset of the event channel interrupts you simply want to
dispatch the irq directly.

Eric
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