Fwd: Re: [Xen-devel] another regression from IRQ handling changes

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

----------  Forwarded Message  ----------

Subject: Re: [Xen-devel] another regression from IRQ handling changes
Date: Tuesday 22 September 2009
From: Keir Fraser <keir.fraser%eu.citrix.com@localhost>
To: Jan Beulich <JBeulich%novell.com@localhost>, 
"xiantao.zhang%intel.com@localhost" 
<xiantao.zhang%intel.com@localhost>

On 22/09/2009 09:53, "Jan Beulich" <JBeulich%novell.com@localhost> wrote:

>> If it wasn't broken before, it was probably broken by Xiantao's follow-up
>> patch to fix NetBSD dom0 (at least as much as possible, to avoid a nasty
>> regression with NetBSD). What we probably need to do is have a 256-entry
>> dom0_vector_to_dom0_irq[] array, and allocate an entry from that for every
>> fresh irq we see at PHYSDEVOP_alloc_irq_vector. Then when the vector gets
>> passed back in on ioapic writes, we index into that array rather than using
>> naked rte.vector.
> 
> Yeah, that's what I would view as the solution to get old functionality
> back. But my question also extended to possible solutions to get beyond
> 256 here, especially such that are also acceptable to the pv-ops Dom0,
> which I'm much less certain about.

Well, we could assume that if we see an irq greater than 256 at
PHYSDEVOP_alloc_irq_vector then the dom0 is dealing in GSIs, and in that
case the 'vector' we return and then gets passed to ioapic_write is rather
redundant. We can work out the GSI from the ioapic rte that is being
modified, after all. So perhaps we could just flip into a non-legacy mode of
operation in that case (perhaps reserve a single magic 'vector' value to
return from PHYSDEVOP_alloc_irq_vector in this case, and if we see that
value in the ioapic write handler then we know to assume that guest pirq ==
gsi).

The legacy case is just to handle NetBSD, which throws non-GSIs at the
PHYSDEVOP_alloc_irq_vector interface, and I doubt it will have worked with
those mega-big systems at any time. So I don't think we're dealing with a
regression there.

 -- Keir



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