KVM: s390: mark irq_state.flags as non-usable

Old kernels did not check for zero in the irq_state.flags field and old
QEMUs did not zero the flag/reserved fields when calling
KVM_S390_*_IRQ_STATE. Let's add comments to prevent future uses of
these fields.

Signed-off-by: Christian Borntraeger <[e-mail]>
Reviewed-by: Thomas Huth <[e-mail]>
Reviewed-by: Cornelia Huck <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Signed-off-by: Christian Borntraeger <[e-mail]>

Documentation/virtual/kvm/api.txt

···
 
 struct kvm_s390_irq_state {
 	__u64 buf;
-	__u32 flags;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 reserved[4];
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
 
 Userspace passes in the above struct and for each pending interrupt a
 struct kvm_s390_irq is copied to the provided buffer.
+
+The structure contains a flags and a reserved field for future extensions. As
+the kernel never checked for flags == 0 and QEMU never pre-zeroed flags and
+reserved, these fields can not be used in the future without breaking
+compatibility.
 
 If -ENOBUFS is returned the buffer provided was too small and userspace
 may retry with a bigger buffer.
···
 
 struct kvm_s390_irq_state {
 	__u64 buf;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 pad;
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
+
+The restrictions for flags and reserved apply as well.
+(see KVM_S390_GET_IRQ_STATE)
 
 The userspace memory referenced by buf contains a struct kvm_s390_irq
 for each interrupt to be injected into the guest.

arch/s390/kvm/kvm-s390.c

···
 // SPDX-License-Identifier: GPL-2.0
 /*
- * hosting zSeries kernel virtual machines
+ * hosting IBM Z kernel virtual machines (s390x)
  *
- * Copyright IBM Corp. 2008, 2009
+ * Copyright IBM Corp. 2008, 2017
  *
  *    Author(s): Carsten Otte <[e-mail]>
  *               Christian Borntraeger <[e-mail]>
···
 			r = -EINVAL;
 			break;
 		}
+		/* do not use irq_state.flags, it will break old QEMUs */
 		r = kvm_s390_set_irq_state(vcpu,
 					   (void __user *) irq_state.buf,
 					   irq_state.len);
···
 			r = -EINVAL;
 			break;
 		}
+		/* do not use irq_state.flags, it will break old QEMUs */
 		r = kvm_s390_get_irq_state(vcpu,
 					   (__u8 __user *)  irq_state.buf,
 					   irq_state.len);

include/uapi/linux/kvm.h

···
 
 struct kvm_s390_irq_state {
 	__u64 buf;
-	__u32 flags;
+	__u32 flags;        /* will stay unused for compatibility reasons */
 	__u32 len;
-	__u32 reserved[4];
+	__u32 reserved[4];  /* will stay unused for compatibility reasons */
 };
 
 /* for KVM_SET_GUEST_DEBUG */