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Re: [Xen-devel] [PATCH 2 of 4] xen: introduce PHYSDEVOP_get_nr_pirqs

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2 of 4] xen: introduce PHYSDEVOP_get_nr_pirqs
From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Date: Tue, 31 Aug 2010 14:05:41 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
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On Tue, 31 Aug 2010, Keir Fraser wrote:
> On 31/08/2010 13:49, "Stefano Stabellini" <Stefano.Stabellini@xxxxxxxxxxxxx>
> wrote:
> >> Couldn't it, like, pick the smallest available? :-)
> >> 
> > 
> > Well, it might still be useful to know the upper limit, besides linux
> > tends to choose the pirq == irq and the irqs for MSIs are high.
> > Xen does the same thing by the way.
> Well I'm just being fussy because it's yet another irq related interface and
> we seem to have so many already. Might-be-useful is different from
> must-have-now, and if you are allocating pirq==irq then that allocation
> strategy is not influenced by knowing nr_pirqs is it?
Knowing the pirq number upper limit is important for PV on HVM guests to
be able to remap MSIs into pirqs minimizing the chances of conflicts.
If we had another way of knowing the maximum pirq number from within
the guest I would gladly use it.
It is also useful for dom0 that up to know just assumed that the pirq
number is always identical to the irq number (that for MSIs might be
actually higher than nr_pirq).
In other words: if the guest is allowed to choose the pirq number it
must be able to know what the range is.
Could we initialize nr_pirq always to the same value in xen?

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