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

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:
>
>   
> > 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.
> >     
> 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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