Re: [Xen-devel] 4.3 regression in booting on a particular platform

[Ben Guthro <ben.guthro@xxxxxxxxx>]

On Jul 10, 2013, at 5:09 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:

>>>> On 09.07.13 at 18:34, Ben Guthro <ben@xxxxxxxxxx> wrote:
>> On Tue, Jul 9, 2013 at 9:18 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>> On 09.07.13 at 14:55, Ben Guthro <ben@xxxxxxxxxx> wrote:
>>>> While this is pre-release hardware from an OEM, it did work on 4.2, so
>>>> I am not (yet) inclined to blame it on being pre-release hardware.
>>>>
>>>> This is very early in boot, and so does not have a proper stack.
>>>> Do you have any thoughts as to where this might be going awry?
>>>
>>> If you could make the xen-syms available matching the xen.gz that
>>> was used when the below log was obtained, I could try to take a
>>> look.
>>>
>>> Perhaps repeating the attempt with "iommu=debug" could reveal
>>> further useful details.
>>
>> I have put xen.gz, the xen-syms, and an updated boot log, with
>> iommu=debug at the following location that you may download:
>> https://citrix.sharefile.com/d/safbf0b05aac4233a
>
> This
>
> (XEN) [VT-D]dmar.c:767: Host address width 39
> (XEN) [VT-D]dmar.c:781: found ACPI_DMAR_DRHD:
> (XEN) [VT-D]dmar.c:461:   dmaru->address = 0
>
> is clearly a BIOS bug, and I don't see how 4.2 would get along
> with that. It may fail in different ways, but that's not directly
> related to how ioremap() is implemented.
>
> That said, the changeset you pointed at uncovered an always
> existing bug in the VT-d code: The mapping of the IOMMU MMIO
> register space is being established through map_to_nocache_virt()
> (resolving to set_fixmap_nocache()), but iommu_free() calls
> iounmap() to undo it (which prior to said changeset wasn't
> doing anything).
>
> Bottom line - this box wants to be booted with "iommu=off" until
> you have a fixed BIOS.

It would be better if we could detect such a buggy bios, and continue
to boot without iommu capability, rather than crash, no?

Thank you for the analysis. Is there a wiki page, or the like that
might give me a pointer on how to do this analysis for myself in the
future, so as not to bother you? (Eg, how to work with Xen-syms)

>
> Jan
>

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