>> So this call amd_iommu_flush_all_devices() will be able to tell devices
>> that don't do any more DMAs and hence it is safe to reprogram iommu
>> mapping entries.
>>
> It blocks the cpu until any pending DMA operations are complete. Hmm, as I
> think about it, there is still a small possibility that a device like a NIC
> which has several buffers pre-dma-mapped could start a new dma before we
> completely disabled the iommu, althought thats small. I never saw that in my
> testing, but hitting that would be fairly difficult I think, since its literally
> just a few hundred cycles between the flush and the actual hardware disable
> operation.
>
> According to this though:
>
http://support.amd.com/us/Processor_TechDocs/34434-IOMMU-Rev_1.26_2-11-09.pdf
> That window could be closed fairly easily, but simply disabling read and write
> permissions for each device table entry prior to calling flush. If we do that,
> then flush the device table, any subsequently started dma operation would just
> get noted in the error log, which we could ignore, since we're abot to boot to
> the kdump kernel anyway.
>
> Would you like me to respin w/ that modification?
