diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 520177f40c17..2efed37753ed 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -2090,22 +2090,29 @@ out_nofid:
  * with qemu_iovec_destroy().
  */
 static void v9fs_init_qiov_from_pdu(QEMUIOVector *qiov, V9fsPDU *pdu,
-                                    size_t skip, size_t size,
+                                    size_t skip, size_t *size,
                                     bool is_write)
 {
     QEMUIOVector elem;
     struct iovec *iov;
     unsigned int niov;
+    size_t alloc_size = *size + skip;
 
     if (is_write) {
-        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, size + skip);
+        pdu->s->transport->init_out_iov_from_pdu(pdu, &iov, &niov, alloc_size);
     } else {
-        pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, size + skip);
+        pdu->s->transport->init_in_iov_from_pdu(pdu, &iov, &niov, &alloc_size);
+    }
+
+    if (alloc_size < skip) {
+        *size = 0;
+    } else {
+        *size = alloc_size - skip;
     }
 
     qemu_iovec_init_external(&elem, iov, niov);
     qemu_iovec_init(qiov, niov);
-    qemu_iovec_concat(qiov, &elem, skip, size);
+    qemu_iovec_concat(qiov, &elem, skip, *size);
 }
 
 static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
@@ -2130,7 +2137,7 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
     }
     offset += err;
 
-    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, read_count, false);
+    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, &read_count, false);
     err = v9fs_pack(qiov_full.iov, qiov_full.niov, 0,
                     ((char *)fidp->fs.xattr.value) + off,
                     read_count);
@@ -2259,9 +2266,11 @@ static void coroutine_fn v9fs_read(void *opaque)
         QEMUIOVector qiov_full;
         QEMUIOVector qiov;
         int32_t len;
+        size_t size = max_count;
 
-        v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset + 4, max_count, false);
+        v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset + 4, &size, false);
         qemu_iovec_init(&qiov, qiov_full.niov);
+        max_count = size;
         do {
             qemu_iovec_reset(&qiov);
             qemu_iovec_concat(&qiov, &qiov_full, count, qiov_full.size - count);
@@ -2504,6 +2513,7 @@ static void coroutine_fn v9fs_write(void *opaque)
     int32_t len = 0;
     int32_t total = 0;
     size_t offset = 7;
+    size_t size;
     V9fsFidState *fidp;
     V9fsPDU *pdu = opaque;
     V9fsState *s = pdu->s;
@@ -2516,7 +2526,9 @@ static void coroutine_fn v9fs_write(void *opaque)
         return;
     }
     offset += err;
-    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, count, true);
+    size = count;
+    v9fs_init_qiov_from_pdu(&qiov_full, pdu, offset, &size, true);
+    count = size;
     trace_v9fs_write(pdu->tag, pdu->id, fid, off, count, qiov_full.niov);
 
     fidp = get_fid(pdu, fid);
diff --git a/hw/9pfs/9p.h b/hw/9pfs/9p.h
index 3904f8290131..8d07a0b301e2 100644
--- a/hw/9pfs/9p.h
+++ b/hw/9pfs/9p.h
@@ -425,7 +425,7 @@ struct V9fsTransport {
     ssize_t     (*pdu_vunmarshal)(V9fsPDU *pdu, size_t offset, const char *fmt,
                                   va_list ap);
     void        (*init_in_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
-                                        unsigned int *pniov, size_t size);
+                                        unsigned int *pniov, size_t *size);
     void        (*init_out_iov_from_pdu)(V9fsPDU *pdu, struct iovec **piov,
                                          unsigned int *pniov, size_t size);
     void        (*push_and_notify)(V9fsPDU *pdu);
diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
index b5a7c03f26d5..991e175c826f 100644
--- a/hw/9pfs/virtio-9p-device.c
+++ b/hw/9pfs/virtio-9p-device.c
@@ -147,19 +147,22 @@ static ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
 }
 
 static void virtio_init_in_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
-                                        unsigned int *pniov, size_t size)
+                                        unsigned int *pniov, size_t *size)
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
     VirtQueueElement *elem = v->elems[pdu->idx];
     size_t buf_size = iov_size(elem->in_sg, elem->in_num);
 
-    if (buf_size < size) {
+    if (buf_size < P9_IOHDRSZ) {
         VirtIODevice *vdev = VIRTIO_DEVICE(v);
 
         virtio_error(vdev,
-                     "VirtFS reply type %d needs %zu bytes, buffer has %zu",
-                     pdu->id + 1, size, buf_size);
+                     "VirtFS reply type %d needs %zu bytes, buffer has %zu, less than minimum",
+                     pdu->id + 1, *size, buf_size);
+    }
+    if (buf_size < *size) {
+        *size = buf_size;
     }
 
     *piov = elem->in_sg;
diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
index 71eebe12dd41..18fe5b7c92fa 100644
--- a/hw/9pfs/xen-9p-backend.c
+++ b/hw/9pfs/xen-9p-backend.c
@@ -187,7 +187,7 @@ static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
 static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU *pdu,
                                           struct iovec **piov,
                                           unsigned int *pniov,
-                                          size_t size)
+                                          size_t *size)
 {
     Xen9pfsDev *xen_9pfs = container_of(pdu->s, Xen9pfsDev, state);
     Xen9pfsRing *ring = &xen_9pfs->rings[pdu->tag % xen_9pfs->num_rings];
@@ -197,16 +197,19 @@ static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU *pdu,
     g_free(ring->sg);
 
     ring->sg = g_new0(struct iovec, 2);
-    xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, size);
+    xen_9pfs_in_sg(ring, ring->sg, &num, pdu->idx, *size);
 
     buf_size = iov_size(ring->sg, num);
-    if (buf_size  < size) {
+    if (buf_size  < P9_IOHDRSZ) {
         xen_pv_printf(&xen_9pfs->xendev, 0, "Xen 9pfs request type %d"
-                "needs %zu bytes, buffer has %zu\n", pdu->id, size,
-                buf_size);
+                "needs %zu bytes, buffer has %zu, less than minimum\n",
+                pdu->id, *size, buf_size);
         xen_be_set_state(&xen_9pfs->xendev, XenbusStateClosing);
         xen_9pfs_disconnect(&xen_9pfs->xendev);
     }
+    if (buf_size  < *size) {
+        *size = buf_size;
+    }
 
     *piov = ring->sg;
     *pniov = num;
