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

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

>>> On 14.12.10 at 09:46, Weidong Han <weidong.han@intel.com> wrote:
> Jan Beulich wrote:
>> 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
>>
>>   
> x2apic spec 2.9 section states: The default will be for the BIOS to pass 
> the control to the OS with the local x2APICs in xAPIC mode if all x2APIC 
> IDs reported by CPUID.0BH:EDX are less than 255, and in x2APIC mode if 
> there are any logical processor reporting its x2APIC ID at 255 or greater.
> 
> If BIOS transitions to Xen with x2APIC enabled, there must be APIC ID > 
> 255, which requires interrupt remapping. So we added code to check such 
> BIOS issue.

So what would you tell a customer of yours reporting native Linux
to boot fine on a certain system, but Xen dying early?

Also, from a purely technical perspective, it doesn't seem like
interrupt re-mapping is a requirement for the BIOS: With
native Linux booting fine, there obviously is no issue. And
clearly, as long as the boot CPU has an APIC id fitting into 8
bits, interrupts ought to work fine without re-mapping.

When the boot CPU has a wider APIC ID, obviously the BIOS
won't be able to bring up the system (and hence the question
of booting whatever OS on top of it would be mute anyway).

Jan