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

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Message-ID: <a3bb0e8e-72c9-4a9c-b347-e899ae29b289@kernel.org>
Date: Fri, 28 Nov 2025 09:10:13 +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 08:49, Ritesh Harjani (IBM) wrote:
> "David Hildenbrand (Red Hat)" <david@...nel.org> writes:
> 
>> 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
> 
> aah right. I should have looked closely. I was thinking you meant
> somewhere in Qemu, so I was only looking there.
> 
>>       
>>       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.
>>
> 
> Right. Let me give it a try first with pseries Qemu. Otherwise I have an
> access to pseries LPAR too. I can verify it there.

Ah, now I realize my problem: I tried with Fedora 43 in a powerpc VM but 
the modprobe didn't do anything.

Looking again:

# grep CONFIG_CMM /boot/config-6.17.1-300.fc43.ppc64le
CONFIG_CMM=y

So modprob'ing won't do as the module is built in.

Anyhow, compiling a kernel inside a QEMU VM might take a while, so if 
you can beat me to it I wouldn't be mad :)


Re powernv vs. powerpc: yeah, it's confusing. For some reason I thought 
I tested it in the days on powernv. Maybe that's simply because I was 
only able to get my hands on such a machine. Looking again, it should 
indeed be able to be simulated on a simple powerpc VM.

-- 
Cheers

David