Re: Seeing DMAR errors after multiple load/unload with SR-IOV

From: padmanabh ratnakar
Date: Tue Jun 07 2011 - 02:23:43 EST

On Tue, Jun 7, 2011 at 4:04 AM, Chris Wright <chrisw@xxxxxxxxxxxx> wrote:
> * Alex Williamson (alex.williamson@xxxxxxxxxx) wrote:
>> On Mon, 2011-06-06 at 14:39 +0530, padmanabh ratnakar wrote:
>> > Hi,
>> >         I am using linux kernel 2.6.39. I have a IBM x3650 M3 system.
>> > I have used following boot options -
>> > intel_iommu=on iommu=pt
>> >
>> > I was loading/unloading my NIC driver(be2net) with num_vfs=7.
>> >
>> > After some iterations I get following DMAR errors -
>> > Jun  4 03:50:20 rhel6 kernel: Uhhuh. NMI received for unknown reason
>> > 2d on CPU 0.
>> > Jun  4 03:50:20 rhel6 kernel: Do you have a strange power saving mode enabled?
>> > Jun  4 03:50:20 rhel6 kernel: Dazed and confused, but trying to continue
>> > Jun  4 03:50:20 rhel6 kernel: DRHD: handling fault status reg 2
>> > Jun  4 03:50:20 rhel6 kernel: DMAR:[DMA Read] Request device [1a:00.2]
>> > fault addr 78077000
>> > Jun  4 03:50:20 rhel6 kernel: DMAR:[fault reason 02] Present bit in
>> > context entry is clear
>> >
>> > I was trying to debug this. I dont understand iommu code much.
>> > The physical address belongs the printed PCI function and there should
>> > not have been an error.
>> >
>> > I am unable to see pci_dev(pdev) of VFs getting removed from
>> > si_domain->devices list(intel-iommu.c)
>> > when driver gets unloaded calling pci_disable_sriov() freeing VF pdevs.
>> > Looks like issue happens when when freed pdev is allocated again and
>> > as it is already in list,
>> > required initializations dont happen.
>> >
>> > I dont know if my understanding is correct. Can anyone point me to
>> > what the issue may be?
> Yes, that's correct.  The (now replaced) check identity_mapping()
> will succeed when the pci_dev is recycled (it's freed, but never
> removed from the list, this is an issue with passtrhough mode and device
> creation/desctruction).  This false match happens w/ a brand new pci_dev
> which still has default 32bit DMA mask, so it is removed from pt domain.
> During removal domain_remove_one_dev_info() test that matches only
> on bus/devfn (now also segment) will match despite the fact that the
> info->pdev != pdev->dev.archdata.iommu.  Then...Oops
>> Typically devices are removed from the domain via
>> drivers/pci/intel-iommu.c:device_notifier(), which is called as the
>> device is unbound from the driver.  However, this seems to get skipped
>> when running in passthrough mode, so I'm not sure where that's supposed
>> to occur.  Does it happen w/o passthrough?
I had tried without passthrough on RHEL 6.1 GA kernel. Was seeing
hangs and panics. Will check if non passthrough mode works on latest kernel.

> If you blacklist the driver then a create/delete may do similar (haven't
> tested that idea).
>> Also note that some
>> intel-iommu fixes have rolled into 3.0.0-rc2, you might want to update
>> and see if anything is better there.  Thanks,
> The change in identity_mapping() means we won't demote to 32-bit DMA
> (drop out of pt domain), so I don't think we'll see the same issue.
For testing I had made a hack in 2.6.39 kernel which will prevent
demoting to 32bit DMA mask
and thereby prevent calling of domain_remove_one_dev_info() for the
specific VF device I was using
and it had worked.
So as you said I may not hit the issue in latest kernel. Will try that.

> thanks,
> -chris

Thanks for the response and suggestions.
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