[RFC PATCH] s390: kvm: reduce frequency of CPU syncs of diag318 info

[Collin Walling <walling@linux.ibm.com>]

DIAGNOSE 0318 is invoked only once during IPL. As such, the
diag318 info will only change once initially and during resets.
Let's only sync the register to convey the info to KVM if and
only if the diag318 info has changed. Only set the dirty bit
flag for diag318 whenever it must be updated.

The migration handler will invoke the set_diag318 helper on
post_load to ensure the info is set on the destination machine.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
---
 target/s390x/kvm/kvm.c |  5 -----
 target/s390x/machine.c | 14 ++++++++++++++
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/target/s390x/kvm/kvm.c b/target/s390x/kvm/kvm.c
index 6acf14d5ec..6a141399f7 100644
--- a/target/s390x/kvm/kvm.c
+++ b/target/s390x/kvm/kvm.c
@@ -599,11 +599,6 @@ int kvm_arch_put_registers(CPUState *cs, int level)
         cs->kvm_run->kvm_dirty_regs |= KVM_SYNC_ETOKEN;
     }
 
-    if (can_sync_regs(cs, KVM_SYNC_DIAG318)) {
-        cs->kvm_run->s.regs.diag318 = env->diag318_info;
-        cs->kvm_run->kvm_dirty_regs |= KVM_SYNC_DIAG318;
-    }
-
     /* Finally the prefix */
     if (can_sync_regs(cs, KVM_SYNC_PREFIX)) {
         cs->kvm_run->s.regs.prefix = env->psa;
diff --git a/target/s390x/machine.c b/target/s390x/machine.c
index 37a076858c..a5d113ce3a 100644
--- a/target/s390x/machine.c
+++ b/target/s390x/machine.c
@@ -234,6 +234,19 @@ const VMStateDescription vmstate_etoken = {
     }
 };
 
+static int diag318_post_load(void *opaque, int version_id)
+{
+    S390CPU *cpu = opaque;
+    CPUState *cs = CPU(cpu);
+    CPUS390XState *env = &S390_CPU(cs)->env;
+
+    if (kvm_enabled()) {
+        kvm_s390_set_diag318(cs, env->diag318_info);
+    }
+
+    return 0;
+}
+
 static bool diag318_needed(void *opaque)
 {
     return s390_has_feat(S390_FEAT_DIAG_318);
@@ -243,6 +256,7 @@ const VMStateDescription vmstate_diag318 = {
     .name = "cpu/diag318",
     .version_id = 1,
     .minimum_version_id = 1,
+    .post_load = diag318_post_load,
     .needed = diag318_needed,
     .fields = (VMStateField[]) {
         VMSTATE_UINT64(env.diag318_info, S390CPU),
-- 
2.31.1