s390x/pv: Fix spurious warning with asynchronous teardown

Kernel commit 292a7d6fca33 ("KVM: s390: pv: fix asynchronous teardown
for small VMs") causes the KVM_PV_ASYNC_CLEANUP_PREPARE ioctl to fail
if the VM is not larger than 2GiB. QEMU would attempt it and fail,
print an error message, and then proceed with a normal teardown.

Avoid attempting to use asynchronous teardown altogether when the VM is
not larger than 2 GiB. This will avoid triggering the error message and
also avoid pointless overhead; normal teardown is fast enough for small
VMs.

Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Fixes: c3a073c ("s390x/pv: Add support for asynchronous teardown for reboot")
Link: https://lore.kernel.org/all/20230421085036.52511-2-imbrenda@linux.ibm.com/
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20230510105531.30623-2-imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
[thuth: Fix inline function parameter in pv.h]
Signed-off-by: Thomas Huth <thuth@redhat.com>

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"
@@ -115,15 +116,20 @@ static void *s390_pv_do_unprot_async_fn(void *p)
      return NULL;
 }
 
-bool s390_pv_vm_try_disable_async(void)
+bool s390_pv_vm_try_disable_async(S390CcwMachineState *ms)
 {
     /*
      * 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)) {
+    /*
+     * If the feature is not present or if the VM is not larger than 2 GiB,
+     * KVM_PV_ASYNC_CLEANUP_PREPARE fill fail; no point in attempting it.
+     */
+    if ((MACHINE(ms)->maxram_size <= 2 * 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) {

hw/s390x/s390-virtio-ccw.c

@@ -330,7 +330,7 @@ static inline void s390_do_cpu_ipl(CPUState *cs, run_on_cpu_data arg)
 
 static void s390_machine_unprotect(S390CcwMachineState *ms)
 {
-    if (!s390_pv_vm_try_disable_async()) {
+    if (!s390_pv_vm_try_disable_async(ms)) {
         s390_pv_vm_disable();
     }
     ms->pv = false;

include/hw/s390x/pv.h

@@ -14,10 +14,10 @@
 
 #include "qapi/error.h"
 #include "sysemu/kvm.h"
+#include "hw/s390x/s390-virtio-ccw.h"
 
 #ifdef CONFIG_KVM
 #include "cpu.h"
-#include "hw/s390x/s390-virtio-ccw.h"
 
 static inline bool s390_is_pv(void)
 {
@@ -41,7 +41,7 @@ static inline bool s390_is_pv(void)
 int s390_pv_query_info(void);
 int s390_pv_vm_enable(void);
 void s390_pv_vm_disable(void);
-bool s390_pv_vm_try_disable_async(void);
+bool s390_pv_vm_try_disable_async(S390CcwMachineState *ms);
 int s390_pv_set_sec_parms(uint64_t origin, uint64_t length);
 int s390_pv_unpack(uint64_t addr, uint64_t size, uint64_t tweak);
 void s390_pv_prep_reset(void);
@@ -61,7 +61,7 @@ static inline bool s390_is_pv(void) { return false; }
 static inline int s390_pv_query_info(void) { return 0; }
 static inline int s390_pv_vm_enable(void) { return 0; }
 static inline void s390_pv_vm_disable(void) {}
-static inline bool s390_pv_vm_try_disable_async(void) { return false; }
+static inline bool s390_pv_vm_try_disable_async(S390CcwMachineState *ms) { return false; }
 static inline int s390_pv_set_sec_parms(uint64_t origin, uint64_t length) { return 0; }
 static inline int s390_pv_unpack(uint64_t addr, uint64_t size, uint64_t tweak) { return 0; }
 static inline void s390_pv_prep_reset(void) {}