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[Xen-devel] MAX_GSI_IRQS vs. MAX_NR_IRQS

To: <xiantao.zhang@xxxxxxxxx>
Subject: [Xen-devel] MAX_GSI_IRQS vs. MAX_NR_IRQS
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Fri, 02 Jul 2010 10:05:27 +0100
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prior to Xen c/s 20072 there was a restriction to the number of
physical IRQs Xen could handle which was tied to the number of
bits that fit in a page. This restriction is needed so that the shared
page IRQ-needs-EOI-notification mechanism can work with a
single page. Your patch, however, changed things so that
MAX_GSI_IRQS now is the number of bits in a page, while
MAX_NR_IRQS is twice that value. From looking over the uses of
the former I would think that this constant can be eliminated
altogether, setting MAX_NR_IRQS to the correct value again and
using it instead of the eliminated constant wherever needed.

The effect of this will be that systems with very many GSIs (how
likely is it that we'll ever see a box with thousands of GSIs?) would
simply not get much room left for IRQs to be used for MSI/MSI-X.

Alternatively MAX_NR_IRQS should be returned back to its original
value, and MAX_GSI_IRQS be set to e.g. half of that value. Such
a restriction seems rather arbitrary to me however, hence a
better alternative would then perhaps be to allow a command line
override just like for nr_irqs.



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