Re: [bug report] vfio: Can't find phys by iova in vfio_unmap_unpin()

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 20 May 2019 15:50:11 +0800
jiangyiwen <jiangyiwen@huawei.com> wrote:

> Hello alex,
> 
> We test a call trace as follows use ARM64 architecture,
> it prints a WARN_ON() when find not physical address by
> iova in vfio_unmap_unpin(), I can't find the cause of
> problem now, do you have any ideas?

Is it reproducible?  Can you explain how to reproduce it?  The stack
trace indicates a KVM VM is being shutdown and we're trying to clean
out the IOMMU mappings from the domain and find a page that we think
should be mapped that the IOMMU doesn't have mapped.  What device(s) was
assigned to the VM?  This could be an IOMMU driver bug or a
vfio_iommu_type1 bug.  Have you been able to reproduce this on other
platforms?

> In addition, I want to know why there is a WARN_ON() instead
> of BUG_ON()? Does it affect the follow-up process?

We're removing an IOMMU page mapping entry and find that it's not
present, so ultimately the effect at the IOMMU is the same, there's no
mapping at that address, but I can't say without further analysis
whether that means a page remains pinned or if that inconsistency was
resolved previously elsewhere.  We WARN_ON because this is not what we
expect, but potentially leaking a page of memory doesn't seem worthy of
crashing the host, nor would a crash dump at that point necessarily aid
in resolving the missing page as it potentially occurred well in the
past.  Thanks,

Alex