[Skiboot] [PATCH] external/mambo: Disable MEMORY_OVERFLOW

Gustavo Romero gromero at linux.vnet.ibm.com
Wed Aug 12 08:00:07 AEST 2020

Hi Oliver,

On 8/4/20 4:01 AM, Oliver O'Halloran wrote:
> On Fri, Jul 3, 2020 at 8:53 AM Gustavo Romero
> <gromero at linux.vnet.ibm.com> wrote:
>> On 7/2/20 4:03 AM, Michael Ellerman wrote:
>>> Gustavo Romero <gromero at linux.vnet.ibm.com> writes:
>>>> On 6/28/20 10:27 PM, Michael Ellerman wrote:
>>>> Today I've built upstream mambo and the same issue happens. I'm clueless
>>>> yet what's happening... so if you have additional things to try let
>>>> me know. It might be an issue with Mambo P10 running on P8.
>>> Just to be clear it doesn't happen with mambo simulating P9 right?
>> Yes, just on mambo P10. P9 is ok. Also, Gleen could not reproduce it
>> w/ mambo P10 running a P9 host (I'm using mambo on a P8 host). I'm not
>> sure if the host matter that much, but I've also tried with the same
>> kernel image and mambo config that Gleen used and I still hit the
>> issue on my P8 host.
>> I'll dig more tomorrow.
> Any updates on this?

So the only update I have is that address in GPR 9 used by dcbt is
indeed correct, it happens that the MMU is already enabled, address
0x0000000000000240 is indeed correct, but will be translated... so
unfortunately not much progress since the last time.

I was only able to reproduce it on a given P8 machine we have in our
lab. Another thing (which doesn't make sense at all) is that it only
reproduces if that P8 machine is running the Ubuntu 18.04.4
4.15.0-110-generic stock kernel, so if I boot the machine
with some upstream one (for instance, 5.5.0) the issue doesn't
reproduce. Nonetheless, the kernel on the host should not affect
Mambo whatsoever. I've spent days digging into that issue but could
not find anything wrong with Mambo or with the kernel booted on
Mambo VM.

Additionally, I've noted that that particular P8 machine is crashing
mysteriously from time to time due to what looks like a HW issue,
so I don't trust it that much at this point.

Glenn was also never able to reproduce it at his side.

Thus, I won't hold the change from Michael anymore, I was caught once
by that Mambo feature being silently enabled when working on a soft/hard
lockup and it was quite misleading. Also, it's not how the real HW works.

So please, go ahead :)


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