Re: [PATCH v5 3/3] target/ppc/kvm: Use host compatibility mode for nested guests

[BALATON Zoltan <balaton@eik.bme.hu>]

On Tue, 4 Aug 2026, Amit Machhiwal wrote:
> On POWER systems, the host CPU may run in a compatibility mode (e.g.,
> a Power11 processor operating in Power10 compatibility mode). When
> running nested KVM guests, QEMU currently derives the host CPU type
> using mfpvr(), which reflects the physical processor version. This can
> result in a mismatch between the CPU model used by QEMU and the
> compatibility mode enforced by the host, leading to guest boot failures
> such as "KVM-NESTEDv2: couldn't set guest wide elements".
>
> Update kvm_ppc_get_host_cpu_class() to check if the host is running in
> a compatibility mode using kvm_ppc_host_compat_pvr(). When available,
> use the compatibility PVR instead of the raw hardware PVR when selecting
> the CPU model. This ensures that QEMU selects a CPU model consistent
> with the host compatibility mode, allowing nested guests to boot
> correctly.
>
> The guard uses #if defined(TARGET_PPC64) to prevent build breakage on
> ppc32 targets where the POWER9/10/11 PVR constants are not defined.
>
> Tested-by: Gautam Menghani <gautam@linux.ibm.com>
> Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> No changes in this version.
>
> target/ppc/kvm.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index d4c5601a00c4..aa8269186a1a 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2682,6 +2682,18 @@ PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)
>     uint32_t host_pvr = mfpvr();
>     PowerPCCPUClass *pvr_pcc;
>
> +#if defined(TARGET_PPC64)
> +#ifndef CONFIG_KVM
> +#error "CONFIG_KVM is not enabled"
> +#endif

In meson.build:

ppc_system_ss.add(when: 'CONFIG_KVM', if_true: files('kvm.c'))

so I think this check and #error does not make sense.

Regards,
BALATON Zoltan

> +    uint32_t compat_host_pvr;
> +
> +    compat_host_pvr = kvm_ppc_host_compat_pvr();
> +    if (compat_host_pvr) {
> +        host_pvr = compat_host_pvr;
> +    }
> +#endif /* TARGET_PPC64 */
> +
>     pvr_pcc = ppc_cpu_class_by_pvr(host_pvr);
>     if (pvr_pcc == NULL) {
>         pvr_pcc = ppc_cpu_class_by_pvr_mask(host_pvr);
>