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 <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index f670e4b9e7f3..57d3ee9e4bde 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -2901,14 +2901,19 @@ userspace buffer and its length:
 
 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.
 
@@ -2932,10 +2937,14 @@ containing a struct kvm_s390_irq_state:
 
 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.
 If one of the interrupts could not be injected for some reason the