Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

[Will Deacon]

Hey Jason,

On Tue, Mar 19, 2024 at 02:50:07PM -0300, Jason Gunthorpe wrote:
> On Tue, Mar 19, 2024 at 03:47:56PM +0000, Will Deacon wrote:
> 
> > Right, it's hard to win if DMA-active devices weren't quiesced properly
> > by the outgoing kernel. Either the SMMU was left in abort (leading to the
> > problems you list above) or the SMMU is left in bypass (leading to possible
> > data corruption). Which is better?
> 
> For whatever reason (and I really don't like this design) alot of work
> was done on x86 so that device continues to work as-was right up until
> the crash kernel does the first DMA operation. Including having the
> crash kernel non disruptively inherit and retain the IOMMU
> configuration. (eg see translation_pre_enabled() stuff in intel
> driver)

Right, I'm also not thrilled about trying to implement that :)
What we have at the moment seems to be good enough to avoid folks
complaining about it.

For the case Tyler is reporting, though, I _think_ it's just a standard
kexec() rather than a crashkernel.

Will