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

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi BALATON,

Thanks for reviewing this patch. Please find my response inline below.

On 2026/08/04 09:55 PM, BALATON Zoltan wrote:
> 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.

You're right that kvm.c is always compiled with CONFIG_KVM enabled due
to the meson.build constraint, so the #ifndef CONFIG_KVM / #error check
can never fire. I added it as an explicit compile-time assertion to
document the assumption, but it is redundant given the build system
already enforces it. I'll drop it in the next version.

Thanks,
Amit

> 
> 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);
> >