[PATCH v2 1/3] hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat

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

During H_CLIENT_ARCHITECTURE_SUPPORT, QEMU attempts to set the requested
CPU compatibility mode via ppc_set_compat_all(). If this fails, the
existing code may fall back to raw mode when supported by the guest.

The current logic treats all failures uniformly and tried to fall back
to raw mode whenever possible. As a result, errors from KVM (e.g.
kvmppc_set_compat() returning -EINVAL) are silently masked, making
debugging difficult and allowing the guest to proceed with an
unsupported or incompatible configuration.

When running with KVM, such failures indicate that the requested
compatibility level is not supported by the host. Do not fallback in
this case; instead, report the error and fail the CAS negotiation.

This makes the failure visible to the user and ensures that invalid
compatibility requests are rejected early rather than being hidden by
fallback behavior. For example, the following error is now reported:

  qemu-system-ppc64: Unable to set CPU compatibility mode in KVM: Invalid argument

Fixes: cc7b35b169e9 ("spapr: fallback to raw mode if best compat mode cannot be set during CAS")
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 hw/ppc/spapr_hcall.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
index 032805a8d0db..de2ea6f5e579 100644
--- a/hw/ppc/spapr_hcall.c
+++ b/hw/ppc/spapr_hcall.c
@@ -1204,6 +1204,15 @@ target_ulong do_client_architecture_support(PowerPCCPU *cpu,
         Error *local_err = NULL;
 
         if (ppc_set_compat_all(cas_pvr, &local_err) < 0) {
+            /*
+             * If KVM rejected the compat mode, do not fallback. This indicates
+             * the host cannot support the requested level.
+             */
+            if (kvm_enabled()) {
+                error_report_err(local_err);
+                return H_HARDWARE;
+            }
+
             /* We fail to set compat mode (likely because running with KVM PR),
              * but maybe we can fallback to raw mode if the guest supports it.
              */
-- 
2.50.1 (Apple Git-155)