Re: [PATCH 2/2] target/ppc/kvm: Use host compatibility mode for nested guests

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

I found a compilation issue with this patch when building for ppc32. The
host PVR constants (CPU_POWERPC_POWER9_DD22, etc.) are only defined under
TARGET_PPC64, and since POWER9/10/11 compatibility modes are 64-bit only,
the code should be guarded with #if defined(TARGET_PPC64). I'll send v2
shortly with this fix.

~Amit

On 2026/04/30 11:43 AM, 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). In such
> cases, the effective CPU level exposed to guests differs from the
> physical processor generation.
> 
> 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.
> 
> In particular, booting a nested guest on a Power11 LPAR configured in
> Power10 compatibility mode fails with errors such as:
> 
>   KVM-NESTEDv2: couldn't set guest wide elements
> 
> This occurs because QEMU selects a CPU model based on the physical
> processor version, while the host operates in a lower compatibility
> mode. As a result, KVM rejects the requested compatibility level during
> guest initialization.
> 
> Add support for querying host compatibility capabilities via the
> KVM_PPC_GET_COMPAT_CAPS ioctl and derive the effective PVR based on the
> compatibility mode reported by KVM. When available, use this
> compatibility PVR instead of the raw hardware PVR when selecting
> the CPU model.
> 
> If the capability is not supported or the query fails, fall back to the
> existing behavior.
> 
> This ensures that QEMU selects a CPU model consistent with the host
> compatibility mode, allowing nested guests to boot correctly.
> 
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>  target/ppc/kvm.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> index 25c28ad089c6..d384161f780b 100644
> --- a/target/ppc/kvm.c
> +++ b/target/ppc/kvm.c
> @@ -2602,11 +2602,63 @@ bool kvmppc_supports_ail_3(void)
>      return cap_ail_mode_3;
>  }
>  
> +static target_ulong kvmppc_get_compat_caps(void)
> +{
> +    struct kvm_ppc_compat_caps host_compat;
> +    target_ulong host_caps;
> +    int ret;
> +
> +    if (!kvm_check_extension(kvm_state, KVM_CAP_PPC_COMPAT_CAPS)) {
> +        return 0;
> +    }
> +
> +    ret = kvm_vm_ioctl(kvm_state, KVM_PPC_GET_COMPAT_CAPS, &host_compat);
> +    if (ret < 0) {
> +        fprintf(stderr, "KVM: failed to get host capabilities\n");
> +        return 0;
> +    }
> +
> +    host_caps = host_compat.compat_capabilities;
> +    return host_caps;
> +}
> +
> +static uint32_t kvm_ppc_host_compat_pvr(void)
> +{
> +    uint32_t compat_host_pvr = 0;
> +    int cap_idx = 0;
> +    target_ulong host_caps = kvmppc_get_compat_caps();
> +
> +    if (host_caps) {
> +        cap_idx = 63 - __builtin_ctzll(host_caps);
> +        switch (cap_idx) {
> +        case H_GUEST_CAP_P9_MODE_BMAP:
> +            compat_host_pvr = CPU_POWERPC_POWER9_DD22;
> +            break;
> +        case H_GUEST_CAP_P10_MODE_BMAP:
> +            compat_host_pvr = CPU_POWERPC_POWER10_DD20;
> +            break;
> +        case H_GUEST_CAP_P11_MODE_BMAP:
> +            compat_host_pvr = CPU_POWERPC_POWER11_DD20;
> +            break;
> +        default:
> +            break;
> +        }
> +    }
> +
> +    return compat_host_pvr;
> +}
> +
>  PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)
>  {
>      uint32_t host_pvr = mfpvr();
> +    uint32_t compat_host_pvr;
>      PowerPCCPUClass *pvr_pcc;
>  
> +    compat_host_pvr = kvm_ppc_host_compat_pvr();
> +    if (compat_host_pvr) {
> +        host_pvr = compat_host_pvr;
> +    }
> +
>      pvr_pcc = ppc_cpu_class_by_pvr(host_pvr);
>      if (pvr_pcc == NULL) {
>          pvr_pcc = ppc_cpu_class_by_pvr_mask(host_pvr);
> -- 
> 2.50.1 (Apple Git-155)
>