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SubjectRe: [PATCH 30 of 38] xen: implement io_apic_ops

* Jeremy Fitzhardinge <> wrote:

> Ingo Molnar wrote:
>> * Jeremy Fitzhardinge <> wrote:
>>> Writes to the IO APIC are paravirtualized via hypercalls, so implement
>>> the appropriate operations.
>>> Signed-off-by: Jeremy Fitzhardinge <>
>>> ---
>>> 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.

it certainly looks thin enough to me although i'm really not sure we
want to virtualize at the IO-APIC level. Peter, what's your


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