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SubjectRe: [PATCH 30 of 38] xen: implement io_apic_ops
Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>
>
>> Writes to the IO APIC are paravirtualized via hypercalls, so implement
>> the appropriate operations.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.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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