arm: enhance kvm_arm_create_scratch_host_vcpu

Support passing NULL for the first parameter (with the same effect
as passing an empty array) and for the third parameter (meaning
that we should not attempt to init the vcpu).

Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1458788142-17509-3-git-send-email-peterx@redhat.com
[PMM: tweaked commit message, comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

2 files changed, 18 insertions, 3 deletions

diff --git a/target-arm/kvm.c b/target-arm/kvm.c
index 969ab0b..3671032 100644
--- a/target-arm/kvm.c
+++ b/target-arm/kvm.c
@@ -62,13 +62,18 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
         goto err;
     }
 
+    if (!init) {
+        /* Caller doesn't want the VCPU to be initialized, so skip it */
+        goto finish;
+    }
+
     ret = ioctl(vmfd, KVM_ARM_PREFERRED_TARGET, init);
     if (ret >= 0) {
         ret = ioctl(cpufd, KVM_ARM_VCPU_INIT, init);
         if (ret < 0) {
             goto err;
         }
-    } else {
+    } else if (cpus_to_try) {
         /* Old kernel which doesn't know about the
          * PREFERRED_TARGET ioctl: we know it will only support
          * creating one kind of guest CPU which is its preferred
@@ -85,8 +90,15 @@ bool kvm_arm_create_scratch_host_vcpu(const uint32_t *cpus_to_try,
         if (ret < 0) {
             goto err;
         }
+    } else {
+        /* Treat a NULL cpus_to_try argument the same as an empty
+         * list, which means we will fail the call since this must
+         * be an old kernel which doesn't support PREFERRED_TARGET.
+         */
+        goto err;
     }
 
+finish:
     fdarray[0] = kvmfd;
     fdarray[1] = vmfd;
     fdarray[2] = cpufd;
diff --git a/target-arm/kvm_arm.h b/target-arm/kvm_arm.h
index 07f0c72..345233c 100644
--- a/target-arm/kvm_arm.h
+++ b/target-arm/kvm_arm.h
@@ -124,9 +124,12 @@ void kvm_arm_reset_vcpu(ARMCPU *cpu);
  * kvm_arm_create_scratch_host_vcpu:
  * @cpus_to_try: array of QEMU_KVM_ARM_TARGET_* values (terminated with
  * QEMU_KVM_ARM_TARGET_NONE) to try as fallback if the kernel does not
- * know the PREFERRED_TARGET ioctl
+ * know the PREFERRED_TARGET ioctl. Passing NULL is the same as passing
+ * an empty array.
  * @fdarray: filled in with kvmfd, vmfd, cpufd file descriptors in that order
- * @init: filled in with the necessary values for creating a host vcpu
+ * @init: filled in with the necessary values for creating a host
+ * vcpu. If NULL is provided, will not init the vCPU (though the cpufd
+ * will still be set up).
  *
  * Create a scratch vcpu in its own VM of the type preferred by the host
  * kernel (as would be used for '-cpu host'), for purposes of probing it