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>

3 files changed, 18 insertions, 7 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
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 34375eed93ee..efa439f6ffb3 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1,8 +1,8 @@
 // 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 <cotte@de.ibm.com>
  *               Christian Borntraeger <borntraeger@de.ibm.com>
@@ -3808,6 +3808,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
                         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);
@@ -3823,6 +3824,7 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
                         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);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 282d7613fce8..496e59a2738b 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -630,9 +630,9 @@ struct kvm_s390_irq {
 
 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 */