spapr: TCG allow up to 8-thread SMT on POWER8 and newer CPUs

PPC TCG supports SMT CPU configurations for non-hypervisor state, so
permit POWER8-10 pseries machines to enable SMT.

This requires PIR and TIR be set, because that's how sibling thread
matching is done by TCG.

spapr's nested-HV capability does not currently coexist with SMT, so
that combination is prohibited (interestingly somewhat analogous to
LPAR-per-core mode on real hardware which also does not support KVM).

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
[ clg: Also test smp_threads when checking for POWER8 CPU and above ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
index 3fd45a6dec..5a0755d34f 100644
--- a/hw/ppc/spapr_caps.c
+++ b/hw/ppc/spapr_caps.c
@@ -473,6 +473,20 @@ static void cap_nested_kvm_hv_apply(SpaprMachineState *spapr,
                 error_append_hint(errp,
                                   "Try appending -machine cap-nested-hv=off\n");
         }
+    } else if (tcg_enabled()) {
+        MachineState *ms = MACHINE(spapr);
+        unsigned int smp_threads = ms->smp.threads;
+
+        /*
+         * Nested-HV vCPU env state to L2, so SMT-shared SPR updates, for
+         * example, do not necessarily update the correct SPR value on sibling
+         * threads that are in a different guest/host context.
+         */
+        if (smp_threads > 1) {
+            error_setg(errp, "TCG does not support nested-HV with SMT");
+            error_append_hint(errp, "Try appending -machine cap-nested-hv=off "
+                                    "or use threads=1 with -smp\n");
+        }
     }
 }