untrusted comment: verify with openbsd-64-base.pub
RWQq6XmS4eDAcSMu71+f62hjd2l2R7FQY5HWSrvL6BiDGP/GKWcMd2zej4FC12YFfczIKo5IQ1eSWJ/tpdg7QxK9Wv/t1uYF9A8=

OpenBSD 6.4 errata 008, November 29, 2018

Writing more than 4GB to a qcow2 volume corrupts the virtual disk.

Apply by doing:
    signify -Vep /etc/signify/openbsd-64-base.pub -x 008_qcow2.patch.sig \
	-m - | (cd /usr/src && patch -p0)

Then rebuild and install vmd:
    cd usr.sbin/vmd
    make obj
    make
    make install

Index: usr.sbin/vmd/vioqcow2.c
===================================================================
RCS file: /cvs/src/usr.sbin/vmd/vioqcow2.c,v
diff -u -p -r1.8 vioqcow2.c
--- usr.sbin/vmd/vioqcow2.c	8 Oct 2018 16:32:01 -0000	1.8
+++ usr.sbin/vmd/vioqcow2.c	26 Nov 2018 07:03:53 -0000
@@ -78,15 +78,15 @@ struct qcdisk {
 	int       fd;
 	uint64_t *l1;
 	off_t     end;
-	uint32_t  clustersz;
+	off_t	  clustersz;
 	off_t	  disksz; /* In bytes */
-	uint32_t cryptmethod;
+	uint32_t  cryptmethod;
 
 	uint32_t l1sz;
 	off_t	 l1off;
 
 	off_t	 refoff;
-	uint32_t refsz;
+	off_t	 refsz;
 
 	uint32_t nsnap;
 	off_t	 snapoff;
@@ -176,7 +176,7 @@ qc2_open(struct qcdisk *disk, int *fds, 
 	struct qcheader header;
 	uint64_t backingoff;
 	uint32_t backingsz;
-	size_t i;
+	off_t i;
 	int version, fd;
 
 	pthread_rwlock_init(&disk->lock, NULL);