[Xen-devel] Re: [PATCH 30 of 38] xen: implement io_apic_ops

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

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