Messages in this thread | | | Date | Thu, 20 Nov 2008 09:00:41 -0800 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 30 of 38] xen: implement io_apic_ops |
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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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