[PATCH v6 3/3] target/ppc/kvm: Use host compatibility mode for nested guests

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 <[email protected]>
Reviewed-by: Gautam Menghani <[email protected]>
Tested-by: Anushree Mathur <[email protected]>
Signed-off-by: Amit Machhiwal <[email protected]>
---
Changes in this version:
- Drop redundant #ifndef CONFIG_KVM / #error guard; kvm.c is only
  compiled when CONFIG_KVM is set (enforced by meson.build), so the
  check can never fire [BALATON]

 target/ppc/kvm.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index d4c5601a00c4..8e89a6c665c8 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -2682,6 +2682,15 @@ PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)
     uint32_t host_pvr = mfpvr();
     PowerPCCPUClass *pvr_pcc;
 
+#if defined(TARGET_PPC64)
+    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);
-- 
2.50.1 (Apple Git-155)