Re: [Xen-devel] RE: [Xen-changelog] [xen-unstable]vt-d: Fixpanic in msi_msg_read_remap_rte with acpi=off

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 19/10/2009 12:51, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> Got a backtrace and Xen boot params? If you pass acpi=off, then
>> disable_acpi() is invoked, and this sets acpi_disabled. If acpi_disabled=1,
>> then iommu_setup() sets iommu_enabled=0. If iommu_enabled=0 then I think all
>> the update_ire_from_* and similar hooks get disabled in the callers. So
>> something unexpected must be happening.
> 
> Hmmm, perhaps this could be an early boot-time crash before iommu_setup() is
> even called? Perhaps moving the if(acpi_disabled) iommu_enabled=0 somewhere
> early-ish in setup.c:__start_xen(), with a warning message, would be the
> right thing to do in that case (e.g., immediately after the call to
> cmdline_parse()).

After some more hmmm'ing I have to admit your original patch was actually
the best way to go. :-)

All other callers of acpi_find_matched_drhd_unit() that you didn't patch
already check for NULL return, so it's presumably expected as a possibility.
And it is okay for msi_msg_{read,write}_remap_rte() to bail silently if they
cannot determine there's any work to do, since they operate as potential
modifiers to operations which are primarily implemented in their callers.

So sorry about that. I'll revert Dexuan's patch.

 -- Keir



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