[PATCH v1 1/1] s390x/pv: Fix spurious warning with asynchronous teardown

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

When rebooting a small VM using asynchronous teardown, a spurious
warning is emitted when the KVM_PV_ASYNC_CLEANUP_PREPARE ioctl fails.

Avoid using asynchronous teardown altogether when the VM is small
enough; the cutoff is set at 4GiB. This will avoid triggering the
warning and also avoid pointless overhead; normal teardown is fast
enough for small VMs.

Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Fixes: c3a073c610 ("s390x/pv: Add support for asynchronous teardown for reboot")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
 hw/s390x/pv.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/hw/s390x/pv.c b/hw/s390x/pv.c
index 49ea38236c..17c5556319 100644
--- a/hw/s390x/pv.c
+++ b/hw/s390x/pv.c
@@ -13,6 +13,7 @@
 
 #include <linux/kvm.h>
 
+#include "qemu/units.h"
 #include "qapi/error.h"
 #include "qemu/error-report.h"
 #include "sysemu/kvm.h"
@@ -117,13 +118,16 @@ static void *s390_pv_do_unprot_async_fn(void *p)
 
 bool s390_pv_vm_try_disable_async(void)
 {
+    MachineState *machine = MACHINE(qdev_get_machine());
     /*
      * t is only needed to create the thread; once qemu_thread_create
      * returns, it can safely be discarded.
      */
     QemuThread t;
 
-    if (!kvm_check_extension(kvm_state, KVM_CAP_S390_PROTECTED_ASYNC_DISABLE)) {
+    /* Avoid the overhead of asynchronous teardown for small machines */
+    if ((machine->maxram_size < 4 * GiB) ||
+        !kvm_check_extension(kvm_state, KVM_CAP_S390_PROTECTED_ASYNC_DISABLE)) {
         return false;
     }
     if (s390_pv_cmd(KVM_PV_ASYNC_CLEANUP_PREPARE, NULL) != 0) {
-- 
2.40.1