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

[Gautam Menghani <gautam@linux.ibm.com>]

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@linux.ibm.com> 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



- 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