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

[Keir Fraser <keir@xen.org>]

On 13/12/2010 08:15, "Jan Beulich" <JBeulich@novell.com> wrote:

>>>> On 11.12.10 at 01:07, "Kay, Allen M" <allen.m.kay@intel.com> wrote:
>> Yes, interrupt remapping is needed to be the intermediary between legacy
>> IOxAPIC and MSI devices and the new x2APIC in the CPU.
> 
> But isn't this only when there are APIC IDs beyond 255?

Apparently not, since even Linux requires irq remapping even when none of
the APIC IDs are greater than 255. Unless running on kvm or xen. I don't
fully understand this particular restriction, mind you.

Actually, my guess is that x2apic mode requires a different format of APIC
message with a 32-bit APICID field, legacy IOxAPIC and MSI devices do not
support the new message format, and so irq remapping hardware is required to
bridge the two formats, even if no actual irq remapping is occurring.

Is that a canny guess, Allen?

 -- Keir

> Jan
> 
>> -----Original Message-----
>> From: Keir Fraser [mailto:keir.xen@gmail.com] On Behalf Of Keir Fraser
>> Sent: Friday, December 10, 2010 10:50 AM
>> To: Kay, Allen M; Jan Beulich; Zhang, Yang Z
>> Cc: xen-devel@lists.xensource.com; Han, Weidong
>> Subject: Re: [Xen-devel] iommu=0 leading to panic when system defaults to
>> using x2apic
>> 
>> Ah, and the interrupt remapping dependency is because PCI(e) devices cannot
>> address 32-bit APIC IDs?
>> 
>>  -- Keir
>> 
>> On 10/12/2010 18:26, "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.
>>> 
>>> Allen
> 
>