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

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Date:	Thu, 20 Nov 2008 13:32:18 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@...p.org> writes:
>
>   
>> 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.
>>     
>
> Bad idea (I think).  We have a 1:1 relationship between the linux irq number and
> the GSI because it makes the code dramatically simpler, and it took significant
> work to get there.  The concept of an intermediate mapping layer sounds nasty.
> But I haven't yet read the patch.
>   

The changes are spread over a number of patches, but the meat of it is 
in "xen: route hardware irqs via Xen".  It turns out fairly simply, but 
perhaps its because I've made a number of simplifying assumptions: 
interrupts are always IOAPIC based, only using ACPI for routing, no MSI 
support yet.

But it seems to me that the only time you really care that the irq isn't 
a gsi is when programming a vector into the ioapics - you need to do a 
irq -> ioapic/pin mapping anyway, so adding a irq -> gsi -> ioapic/pin 
map isn't all that complex.  And conversely, when probing devices you 
need to map gsi->irq to see whether the interrupt is shared, though you 
could do that on a pure gsi level anyway.

And of course the current code isn't purely irq == gsi anyway, since 
msis are allocated irqs as well, and there's no underlying gsi.  In a 
sense you can think of the other Xen interrupt sources as being a bit 
like MSI, at least in as much as they're not sourced from a GSI (but 
they go further and are not sourced from an IOAPIC at all).

    J
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