spapr: Clarify error and documentation for broken KVM XICS

When starting an L2 KVM guest with `ic-mode=dual,kernel-irqchip=on`,
QEMU fails with:

KVM is too old to support ic-mode=dual,kernel-irqchip=on

This error message was introduced to detect older KVM versions that
didn't allow destruction and re-creation of the XICS KVM device that
we do at reboot. But it is actually the same issue that we get with
nested guests : when running under pseries, KVM currently provides
a genuine XICS device (not the XICS-on-XIVE device that we get
under powernv) which doesn't support destruction/re-creation.

This will eventually be fixed in KVM but in the meantime, update
the error message and documentation to mention the nested case.
While here, mention that in "No XIVE support in KVM" section that
this can also happen with "guest OSes supporting XIVE" since
we check this at init time before starting the guest.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1890290
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <159664243614.622889.18307368735989783528.stgit@bahia.lan>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 9 insertions, 3 deletions

diff --git a/hw/ppc/spapr_irq.c b/hw/ppc/spapr_irq.c
index 2f8f7d62f8..72bb938375 100644
--- a/hw/ppc/spapr_irq.c
+++ b/hw/ppc/spapr_irq.c
@@ -139,6 +139,7 @@ SpaprIrq spapr_irq_dual = {
 
 static int spapr_irq_check(SpaprMachineState *spapr, Error **errp)
 {
+    ERRP_GUARD();
     MachineState *machine = MACHINE(spapr);
 
     /*
@@ -179,14 +180,19 @@ static int spapr_irq_check(SpaprMachineState *spapr, Error **errp)
 
     /*
      * On a POWER9 host, some older KVM XICS devices cannot be destroyed and
-     * re-created. Detect that early to avoid QEMU to exit later when the
-     * guest reboots.
+     * re-created. Same happens with KVM nested guests. Detect that early to
+     * avoid QEMU to exit later when the guest reboots.
      */
     if (kvm_enabled() &&
         spapr->irq == &spapr_irq_dual &&
         kvm_kernel_irqchip_required() &&
         xics_kvm_has_broken_disconnect(spapr)) {
-        error_setg(errp, "KVM is too old to support ic-mode=dual,kernel-irqchip=on");
+        error_setg(errp,
+            "KVM is incompatible with ic-mode=dual,kernel-irqchip=on");
+        error_append_hint(errp,
+            "This can happen with an old KVM or in a KVM nested guest.\n");
+        error_append_hint(errp,
+            "Try without kernel-irqchip or with kernel-irqchip=off.\n");
         return -1;
     }