linux-kernel - Re: [PATCH v1 0/2] powerpc/pseries/cmm: two smaller fixes

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Message-ID: <3cc7d401-c77c-4ac8-abb8-c264fbd9aaf0@kernel.org>
Date: Fri, 28 Nov 2025 08:36:06 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH v1 0/2] powerpc/pseries/cmm: two smaller fixes

On 11/28/25 06:05, Ritesh Harjani (IBM) wrote:
> "David Hildenbrand (Red Hat)" <david@...nel.org> writes:
> 
>> On 10/21/25 12:06, David Hildenbrand wrote:
>>> Two smaller fixes identified while doing a bigger rework.
>>>
>>> Compile-tested only as I don't have an easy way to test right now.
>>>
>>> I would prefer for these patches to go through the MM tree as I will
>>> be sending out a bigger version soon that is based on this series --
>>> I split of the fixes from the other stuff.
>>>
>>
>> Ping,
>>
>> I tried to get CMM running again (using the simulation mode I introduced
>> a while back), but so far my attempts to get a reasonable powernv VM
>> booted in QEMU failed :(
>>
>> (e.g., Fedora qcow2 images use xfs, but the open powernv loader is based
>> on a 5.10 kernel without some mystical XFS feature ...)
>>
> 

Hi Ritesh,

> Sorry, I missed seeing this earlier.
> 
> Do you have the link to simulation mode which you are referring above
> please? So far I didn't find the support of this beyond Linux LPAR
> (pseries), but maybe I missed it.

When I did a rework of the CMM balloon in 2019, I needed a way to test
it. So I added

commit b1713975c31ae20ecc40fd00191ee3fa51445d4a
Author: David Hildenbrand <david@...nel.org>
Date:   Thu Oct 31 15:29:31 2019 +0100

     powerpc/pseries/cmm: Simulation mode
     
     Let's allow to test the implementation without needing HW support.
     When "simulate=1" is specified when loading the module, we bypass all
     HW checks and HW calls. The sysfs file "simulate_loan_target_kb" can
     be used to simulate HW requests.
     
     The simualtion mode can be activated using:
       modprobe cmm debug=1 simulate=1
     
     And the requested loan target can be changed using:
       echo X > /sys/devices/system/cmm/cmm0/simulate_loan_target_kb


I allows for bypassing the absence of FW_FEATURE_CMO.

Back in the days I was able to test it with a pseries (or was it powernv?)
machine we had here at RH. So far my attempts failed to test it in a
similar fashion in QEMU.

-- 
Cheers

David