Subject: Re: [PATCH 2/2] pseries/iommu: remove DDW on kexec

From: Nishanth Aravamudan <[email-masked]>
Date: Tue, 29 Jan 2013 12:33:46 -0800

Hi Michael,

On 29.01.2013 [21:58:28 +1100], Michael Ellerman wrote:
> On Mon, 2013-01-28 at 18:03 -0800, Nishanth Aravamudan wrote:
> > pseries/iommu: remove DDW on kexec
> >  ...
> >     
> > I believe the simplest, easiest-to-maintain fix is to just change our
> > initcall to, rather than detecting and updating the new kernel's DDW
> > knowledge, just remove all DDW configurations. When the drivers
> > re-initialize, we will set everything back up as it was before.
> 
> I don't know this code at all, but this sounds like it will also work
> for kdump, right? ie. when the original kernel has crashed the 2nd
> kernel will tear the DDW down and set it back up.

Yes, my actual test-case (and what was reported as broken) was kdump.
>>From my relatively vague (but now growing) understanding of that
process, kdump does use kexec under the covers to switch to the crash
kernel, and so does get the same benefit from this change.

Another datapoint, though, is that it might make sense to recommend (and
I'm working on figuring this out for the distros, etc) to use
disable_ddw anyways for the kdump kernel command-line, as DDW isn't
'free' and it's unclear if performance is a huge concern for the crash
kernel (sort of varies with where your storage is, and how much you need
to dump, which for kdump generally doesn't seem like that much?).

Thanks,
Nish