s390x/pv: Add support for asynchronous teardown for reboot

This patch adds support for the asynchronous teardown for reboot for
protected VMs.

When attempting to tear down a protected VM, try to use the new
asynchronous interface first. If that fails, fall back to the classic
synchronous one.

The asynchronous interface involves invoking the new
KVM_PV_ASYNC_DISABLE_PREPARE command for the KVM_S390_PV_COMMAND ioctl.

This will prepare the current protected VM for asynchronous teardown.
Once the protected VM is prepared for teardown, execution can continue
immediately.

Once the protected VM has been prepared, a new thread is started to
actually perform the teardown. The new thread uses the new
KVM_PV_ASYNC_DISABLE command for the KVM_S390_PV_COMMAND ioctl. The
previously prepared protected VM is torn down in the new thread.

Once KVM_PV_ASYNC_DISABLE is invoked, it is possible to use
KVM_PV_ASYNC_DISABLE_PREPARE again. If a protected VM has already been
prepared and its cleanup has not started, it will not be possible to
prepare a new VM. In that case the classic synchronous teardown has to
be performed.

The synchronous teardown will now also clean up any prepared VMs whose
asynchronous teardown has not been initiated yet.

This considerably speeds up the reboot of a protected VM; for large VMs
especially, it could take a long time to perform a reboot with the
traditional synchronous teardown, while with this patch it is almost
immediate.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230214163035.44104-3-imbrenda@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/s390x/pv.c

@@ -16,6 +16,7 @@
 #include "qapi/error.h"
 #include "qemu/error-report.h"
 #include "sysemu/kvm.h"
+#include "sysemu/cpus.h"
 #include "qom/object_interfaces.h"
 #include "exec/confidential-guest-support.h"
 #include "hw/s390x/ipl.h"
@@ -108,6 +109,33 @@ void s390_pv_vm_disable(void)
      s390_pv_cmd_exit(KVM_PV_DISABLE, NULL);
 }
 
+static void *s390_pv_do_unprot_async_fn(void *p)
+{
+     s390_pv_cmd_exit(KVM_PV_ASYNC_CLEANUP_PERFORM, NULL);
+     return NULL;
+}
+
+bool s390_pv_vm_try_disable_async(void)
+{
+    /*
+     * 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)) {
+        return false;
+    }
+    if (s390_pv_cmd(KVM_PV_ASYNC_CLEANUP_PREPARE, NULL) != 0) {
+        return false;
+    }
+
+    qemu_thread_create(&t, "async_cleanup", s390_pv_do_unprot_async_fn, NULL,
+                       QEMU_THREAD_DETACHED);
+
+    return true;
+}
+
 int s390_pv_set_sec_parms(uint64_t origin, uint64_t length)
 {
     struct kvm_s390_pv_sec_parm args = {

hw/s390x/s390-virtio-ccw.c

@@ -41,6 +41,7 @@
 #include "hw/qdev-properties.h"
 #include "hw/s390x/tod.h"
 #include "sysemu/sysemu.h"
+#include "sysemu/cpus.h"
 #include "hw/s390x/pv.h"
 #include "migration/blocker.h"
 #include "qapi/visitor.h"
@@ -329,7 +330,9 @@ static inline void s390_do_cpu_ipl(CPUState *cs, run_on_cpu_data arg)
 
 static void s390_machine_unprotect(S390CcwMachineState *ms)
 {
-    s390_pv_vm_disable();
+    if (!s390_pv_vm_try_disable_async()) {
+        s390_pv_vm_disable();
+    }
     ms->pv = false;
     migrate_del_blocker(pv_mig_blocker);
     error_free_or_abort(&pv_mig_blocker);

include/hw/s390x/pv.h

@@ -41,6 +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);
 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);
@@ -60,6 +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 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) {}