Re: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF

[Robin Murphy <robin.murphy@arm.com>]

On 2020-06-01 22:28, Mathieu Poirier wrote:
> That being said I'm sure that dependencies on an IOMMU isn't a problem confined
> to coresight. I am adding Robin Murphy, who added this commit [1], to the thread
> in the hope that he can provide guidance on the right way to do this.

Right, it's not specific to CoreSight, and it's not even specific to 
IOMMUs really. In short, blame kexec ;)

The fundamental thing is that devices should stop any DMA activity at 
shutdown. For a normal poweroff you can typically get away without doing 
so, but over kexec, ongoing DMA traffic may corrupt memory in the new 
kernel (at worst, I think even DMA reads could potentially cause 
unexpected cache behaviour that might lead to mishaps, given the right 
combination of memory attributes).

IOMMUs merely help to make the situation more serious. For similar kexec 
reasons, they need to disable any existing translations at shutdown 
(imagine if the second kernel didn't have an IOMMU driver). And at that 
point, even the normal poweroff case becomes problematic, because any 
device DMA that hasn't been shut down beforehand is now not necessarily 
going benignly to memory as it would in the no-IOMMU case above, but 
potentially to random physical addresses, with all the hilarity ensuing 
that you would expect from that.

Robin.