Re: ACPI-Tables corrupted?

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

On 07/28/2010 02:45 PM, Keir Fraser wrote:
> On 28/07/2010 13:13, "Juergen Gross"<juergen.gross@ts.fujitsu.com>  wrote:
>
>>>> As Dom0 is a pv-kernel, it should be able to ignore this entry.
>>>> The crash kernel OTOH should not panic due to the trashed entry!
>>>> What is the correct solution here?
>>>
>>> Could provide a cmdline option to not nobble the DMAR?
>>
>> That's a possibility.
>> I wonder whether it wouldn't be better to let dom0 decide not to use it if
>> running under xen. This would remove the requirement for zapping the ACPI
>> table. IMO it's always a bad idea to change data of a deeper layer...
>
> If we don't zap the DMAR then every existing dom0 kernel will fail with new
> hypervisor. We could gate it on a new elfnote, or rename to XMAR and have
> dom0 rename it back, or just have a flag day.

The really clean solution would be to virtualize the ACPI table for dom0 and
remove the DMAR entry in this version. This would require some major work, I
guess (clone at least the BIOS page containing the ACPI anchor and present
a modified version to dom0).

>
>>>> The crash kernel expects a valid DMAR entry, as following code in
>>>> enable_IR_x2apic() suggests:
>>>
>>> I don't know what that function does, nor how the error path below depends
>>> on DMAR. DMAR isn't mentioned in the below code.
>>
>> Sorry, here a larger fragment (source arch/x86/kernel/apic/apic.c):
>>                   /* IR is required if there is APIC ID>  255 even when running
>>                    * under KVM
>>                    */
>>                   if (max_physical_apicid>  255 || !kvm_para_available())
>>                           goto nox2apic;
>
> The if stmt is confusing. Also, what would happen if this kernel was booted
> on a system without VT-d (and hence no DMAR)? Presumably it *can* boot in a
> DMAR-less environment -- there must be something odd going on for it to end
> on this path for us.

Yeah, that puzzled me, too.


Juergen

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Juergen Gross                 Principal Developer Operating Systems
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