[Skiboot] (no subject)

Alexey Kardashevskiy aik at ozlabs.ru
Sat Dec 2 11:20:20 AEDT 2017

On 02/12/17 11:13, Nicholas Piggin wrote:
> On Fri, 01 Dec 2017 14:51:56 -0600
> Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:
>> On Fri, 2017-12-01 at 01:52 +1000, Nicholas Piggin wrote:
>>> Firstly, Linux should set up MMU registers like PIDR properly in
>>> its per-CPU mmu initialisation at boot, patch for that should not
>>> be controversial.  
>> So I was wondering how we got things into the PWC since we are 
>> in real mode when we do the flush, but the above explains it.
>> We come up with a stale PIDR and turn the MMU on. We only
>> execute/load/store from Q3 but prefetch/speculation can hit Q0 and thus
>> get crap into the PWC.
>> So I think setting PIDR to 0 is the main fix. Cleaning up the rest also
>> makes sense of course.
>> Or am I missing something else still ?
> Yes that's what happens. After realizing this, the bug no longer requires
> translations to be cached in real mode (does the architecture guarantee
> that?)
> I think LPID is safer to be zeroed as well because in theory we might
> get speculative access into quadrant 0/1.

Is there something quick to try?

I am having a problem with kdump not working while kexec still works just
fine with the exact same kernel, and with kdump triggered by "echo c >
/proc/sysrq-trigger", the new kernel crashes silently somewhere here:



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