Re: [PATCH] KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode

[Vaibhav Jain]

Gautam Menghani <gautam@xxxxxxxxxxxxx> writes:

> On Fri, Jun 05, 2026 at 12:55:50PM +0530, Vaibhav Jain wrote:
>> Hi Gautam,
>> 
>> Thanks for testing this patch. Few questions:
>> Gautam Menghani <gautam@xxxxxxxxxxxxx> writes:
>> 
>> > On Wed, Jun 03, 2026 at 07:45:39PM +0530, Amit Machhiwal wrote:
>> >> On IBM POWER systems, newer processor generations can operate in
>> >> compatibility modes corresponding to earlier generations. This becomes
>> >> relevant for nested virtualization, where nested KVM guests may need to
>> >> run with a specific processor compatibility level.
>> >> 
>> <snip>
>> >
>> > I booted a KVM guest on LPAR with this patch in the following scenarios:
>> <snip>
>> 
>> > 3. P11 guest on P11 host booted in P10 compat mode: No error observed
>> This should have resulted in an error since booting a P11 guest on P10
>> compat mode host is not allowed with/without this patch. Can you please
>> check your test env and share the boot results.
>
> - lscpu output (host P11 LPAR booted in p10 compat mode)
> # lscpu                                                                                                                                                   03:35:13 [3/3]
> Architecture:                ppc64le
>   Byte Order:                Little Endian
> CPU(s):                      960
>   On-line CPU(s) list:       0-959
> Model name:                  POWER10 (architected), altivec supported
>   Model:                     2.0 (pvr 0082 0200)
>   Thread(s) per core:        8
>   Core(s) per socket:        15
>   Socket(s):                 8
>   Physical sockets:          4
>   Physical chips:            2
>   Physical cores/chip:       16
>
>
> - lscpu output from guest
> # lscpu
> Architecture:             ppc64le
>   Byte Order:             Little Endian
> CPU(s):                   4
>   On-line CPU(s) list:    0-3
> Model name:               Power11 (raw), altivec supported
>   Model:                  2.0 (pvr 0082 0200)
>   Thread(s) per core:     1
>   Core(s) per socket:     4
>   Socket(s):              1
>
Argh, this doesnt look right. The kernel patch should have prevented the
P11 compat guest boot on P10 compat host. Looks like you havent used the
corrosponding Qemu patch [1] that could have prevented this from
happening.

Had a off mailing list discussion with Amit on how to address this issue
and he will be sending a new version of the patch to address this issue.


[1] https://lore.kernel.org/all/20260502140021.69712-2-amachhiw@xxxxxxxxxxxxx/

>
>
> - QEMU command line
> /usr/bin/qemu-system-ppc64 -device virtio-blk-pci,drive=drive0,id=virtblk0 \
>     -drive file=/home/gautam/images/fc41.qcow2,format=qcow2,if=none,id=drive0 \
>     -m 100G -smp 4 -cpu host -nographic -machine pseries,ic-mode=xics -accel kvm
>
>
> Thanks,
> Gautam
>

-- 
Cheers
~ Vaibhav