linux-kernel - Re: [PATCH 30 of 38] xen: implement io_apic

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Date:	Thu, 20 Nov 2008 20:27:21 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Xen-devel <xen-devel@...ts.xensource.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH 30 of 38] xen: implement io_apic_ops

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

>
> Yes, I suppose we can statically partition the irq space.  In fact the original
> 2.6.18-xen dom0 kernel does precisely that, but runs into limitations because of
> the compile-time limit on NR_IRQS in that kernel.  If we move to a purely
> dynamically allocated irq space, then having a sparse allocation if irqs becomes
> reasonable again, for msis and vectorless Xen interrupts.
>
>> The difference is that the xen sources are not delivered using vectors.  The cpu
>> vector numbers we do hide and treat as an implementation detail.  And I am totally
>> happy not going through the vector allocation path.
>>
>
> Right.  And in the physical irq event channel case, the vector space is managed
> by Xen, so we need to use Xen to allocate the vector, then program that into the
> appropriate place in the ioapic.

We should be able to share code with iommu for irqs handling, at first glance you
are describing a pretty similar problem.  Now I don't know think the interrupt
remapping code is any kind of beauty but that seems to be roughly what you
are doing with Xen domU.  I certainly think with some careful factoring
we can share the ioapic munging code.  And the code to pick how we program
the ioapics.

>> My gut feel says that you just want to use a different set of irq operations when
>> doing Xen native and working with hardware interrupts.  I haven't seen the code so
>> I don't know how you interact there.  Except in dom0 this is not a consideration so
>> I don't how it is handled.
>>
>
> Yeah.  In the domU case, where there's no physical interrupts, the Xen code
> completely avoids the ioapic/vector stuff, and directly converts an event
> channel into an irq.  Indeed, physical irq delivery is handled the same way; its
> just that the setup requires touching the ioapics to program the appropriate
> vector and bind it to an event channel.

Reasonable.  A lot like the intel interrupt remapping code in that respect.  The
message we program in has little to do with the vector the interrupt arrives on.

So I don't quite know where to hook it but if we are careful we should be able
to get a good interrupt mapping.

Eric
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