Re: [Xen-devel] default value of extra_dom0_irqs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.11.11 at 04:40, Shu Wu <superwushu@xxxxxxxxx> wrote:
> In the changes I noticed the extra_dom0_irqs, which should be 0 by default
> in r20142, was set to 256 in r20143, and caused default number of Dom0's
> nr_pirq to exceed 256. Maybe this prevent IRQ of HP RAID controller, I
> don't quite know about, though. After I set it to 32 (the same number as
> extra_guest_irqs) the cciss.ko worked well. Although I could set this value
> by "extra_guest_irqs=32,32" in boot param, there are still problem:

That would hint at the IRQ number (presumably an MSI one) getting
stored in too narrow a field somewhere in the kernel.

> 1. The argument for dom0 extra irqs, the one after the comma, is
> undocumented.

Feel free to submit a patch to update the respective documentation.
But for your purpose you don't even need the second value afaiu.

> 2. What is the reason of the magic number 256 for Dom0, and 32 for DomU in
> Xen 4.x by default?

They're not magic in any way; if they're found to be too small for a
significant portion of systems, they could get bumped (but not
lowered).

> nr_irqs_gsi is only 16 on x64 arch, but the total

That you speak of one particular system. Most that I work with have
larger values.

> nr_pirq would be more than 256. The magic number still exists in the newest
> code. This is bad hardcode and may cause very elusive fault for newbie
> user, maybe you can have a better solution.

The problem is that we can't judge reasonable for everyone values
here. As long as they serve a majority, we're fine with requiring the
few remaining systems to be run with a command line override.

Speaking of which, one option possible after work that happened over
the last couple of months would be to get rid of ->nr_pirqs altogether,
using nr_irqs again instead. That would make things only worse for your
case though, as you wouldn't then have a way to override the system
determined values.

Jan


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