Xen devel - Jan 2010 - iommu=1 by default?

Would it make sense to make "iommu=1" the default for xen 4.0?

I think Xen is rather more defensive these days about disabling iommu in the
presence of broken BIOS tables, but there may conceivably still be machines it
breaks boot on. I can''t think of any subtle bugs the IOMMU could
introduce if it''s not explicitly in use by VMs -- and I don''t
believe anyone has seen any performance issues. [We could actually not fully
turn the IOMMU on until the first VM that demands it.]

Would it make sense to turn it on by default in the release candidates? Or too
late?

IOMMU is clearly going to be a widely used feature in this release.

Ian

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On 19/01/2010 09:59, "Ian Pratt" <Ian.Pratt@eu.citrix.com>
wrote:
> Would it make sense to make "iommu=1" the default for xen 4.0?
> 
> I think Xen is rather more defensive these days about disabling iommu in
the
> presence of broken BIOS tables, but there may conceivably still be machines
it
> breaks boot on. I can''t think of any subtle bugs the IOMMU could
introduce if
> it''s not explicitly in use by VMs -- and I don''t believe
anyone has seen any
> performance issues. [We could actually not fully turn the IOMMU on until
the
> first VM that demands it.]
> 
> Would it make sense to turn it on by default in the release candidates? Or
too
> late? 
> 
> IOMMU is clearly going to be a widely used feature in this release.
We could enable by default in -rc2 and then disable for final release if
there are bug reports that are not fixable in time.

 -- Keir




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