lkml.org 
[lkml]   [2020]   [Jun]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 2/2] coresight: tmc: Add shutdown callback for TMC ETR/ETF
From
Date
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.

\
 
 \ /
  Last update: 2020-06-03 13:37    [W:0.132 / U:0.100 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site