KVM: s390: add capability indicating COW support

Currently qemu/kvm on s390 uses a guest mapping that does not
allow the guest backing page table to be write-protected to
support older systems. On those older systems a host write
protection fault will be delivered to the guest.

Newer systems allow to write-protect the guest backing memory
and let the fault be delivered to the host, thus allowing COW.

Use a capability bit to tell qemu if that is possible.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/s390/kvm/kvm-s390.c

@@ -28,6 +28,7 @@
 #include <asm/pgtable.h>
 #include <asm/nmi.h>
 #include <asm/switch_to.h>
+#include <asm/sclp.h>
 #include "kvm-s390.h"
 #include "gaccess.h"
 
@@ -140,6 +141,9 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_MAX_VCPUS:
 		r = KVM_MAX_VCPUS;
 		break;
+	case KVM_CAP_S390_COW:
+		r = sclp_get_fac85() & 0x2;
+		break;
 	default:
 		r = 0;
 	}