RE: iommu=0 leading to panic when system defaults to using x2apic

["Jan Beulich" <JBeulich@novell.com>]

>>> On 10.12.10 at 19:26, "Kay, Allen M" <allen.m.kay@intel.com> wrote:
> The architectural requirement is actually between interrupt remapping and 
> x2apic.  Since interrupt remapping is part of the VT-d feature so current 
> software requires all VT-d features enabled in order for x2apic to be enabled.
> 
> Strictly speaking DMA remapping is not required for x2apic.  However, queued 
> invalidation is required since interrupt remapping requires queued 
> invalidation.  So x2apic dependency is as follows:
> 
>     x2apic->interrupt remapping->queued invalidation
> 
> Due to historical reasons, the new VT-d features were built on top of the old 
> ones as they become available.  Is there a requirement to separate this out?  
> If so, we will need to re-design iommu boot parameter which took a while to 
> get it right so most systems can now boot successfully.

And why is it that Xen panics when the BIOS pre-enabled x2apic
mode without also enabling interrupt re-mapping, while Linux
(afaict from looking at 2.6.32 code and output from an affected
machine) simply enables interrupt re-mapping in this case?

Thanks, Jan