RFC on Kdump and PCIe on ARM64

[Bjorn Helgaas]

[+cc Joerg, David, iommu list]

On Thu, Mar 01, 2018 at 12:44:26PM -0500, Sinan Kaya wrote:
> Hi,
> 
> We are seeing IOMMU faults when booting the kdump kernel on ARM64.
> 
> [    7.220162] arm-smmu-v3 arm-smmu-v3.0.auto: event 0x02 received:
> [    7.226123] arm-smmu-v3 arm-smmu-v3.0.auto:            0x0000010000000002
> [    7.232023] arm-smmu-v3 arm-smmu-v3.0.auto:            0x0000000000000000
> [    7.237925] arm-smmu-v3 arm-smmu-v3.0.auto:            0x0000000000000000
> [    7.243827] arm-smmu-v3 arm-smmu-v3.0.auto:            0x0000000000000000
> 
> This is Nate's interpretation of the fault:
> 
> "The PCI device is sending transactions just after the SMMU was
> reset/reinitialized which is problematic because the device has not
> yet been added to the SMMU and thus should not be doing *any* DMA.
> DMA from the PCI devices should be quiesced prior to starting the
> crashdump kernel or you risk overwriting portions of memory you
> meant to preserve. In this case the SMMU was actually doing you a
> favor by blocking these errant DMA operations!!"
> 
> I think this makes sense especially for the IOMMU enabled case on
> the host where an IOVA can overlap with the region of memory kdump
> reserved for itself.
> 
> Apparently, there has been similar concerns in the past.
> 
> https://www.fujitsu.com/jp/documents/products/software/os/linux/catalog/LinuxConJapan2013-Indoh.pdf
> 
> and was not addressed globally due to IOMMU+PCI driver ordering
> issues and bugs in HW due to hot reset.
> 
> https://lkml.org/lkml/2012/8/3/160
> 
> Hot reset as mentioned is destructive and may not be the best
> implementation choice.  However, most of the modern endpoints
> support PCIE function level reset.
> 
> One other solution is for SMMUv3 driver to reserve the kdump used
> IOVA addresses.
> 
> Another solution is for the SMMUv3 driver to disable PCIe devices
> behind the SMMU if it see SMMU is already enabled.

What problem are you trying to solve?  If the IOMMU is blocking DMA
after the kdump kernel starts up, that sounds like the desired
behavior.