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

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Date:	Thu, 20 Nov 2008 09:00:41 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	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>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 30 of 38] xen: implement io_apic_ops

Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>
>   
>> Writes to the IO APIC are paravirtualized via hypercalls, so implement
>> the appropriate operations.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
>> ---
>>  arch/x86/xen/Makefile    |    3 +-
>>  arch/x86/xen/apic.c      |   66 ++++++++++++++++++++++++++++++++++++++++++++++
>>  arch/x86/xen/enlighten.c |    2 +
>>  arch/x86/xen/xen-ops.h   |    2 +
>>  4 files changed, 72 insertions(+), 1 deletion(-)
>>     
>
> hm, why is the ioapic used as the API here, and not an irqchip?
>   

In essence, the purpose of the series is to break the 1:1 relationship 
between Linux irqs and hardware GSIs.  This allows me to have my own irq 
allocator, which in turn allows me to intermix "physical" irqs (ie, a 
Linux irq number bound to a real hardware interrupt source) with the 
various software/virtual irqs the Xen system needs.

Once a physical irq has been mapped onto a gsi interrupt source, the 
mechanisms for handing the ioapic side of things are more or less the 
same.  There's the same procedure of finding the ioapic/pin for a gsi 
and programming the appropriate vector.

(Presumably once I implement MSI support, all references to "gsi" will 
become "gsi/msi/etc".)

So, there's an awkward tradeoff.  I could just completely duplicate the 
whole irq/vector/ioapic management code and hide it under my own 
irqchip, but it would end up duplicating a lot of the existing code.  My 
alternative was to try to open out the existing code into something like 
a thin ioapic library, which I can call into as needed.  The only 
low-level difference is that the Xen ioapics need to be programmed via a 
hypercall rather than register writes.

If the x86 interrupt layer in general decouples irqs from GSIs, then I 
can probably make use of that to clean things up.  A general irq 
allocator along with some way of attaching interrupt-source-specific 
information to each irq would get me a long way, I think.  I'd still 
need hooks to paravirtualize the actual ioapic writes, but at least I 
wouldn't need to have quite so much delicate hooking.

Thanks,
    J
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