virtio: move allocation to virtqueue_pop/vring_pop

The return code of virtqueue_pop/vring_pop is unused except to check for
errors or 0.  We can thus easily move allocation inside the functions
and just return a pointer to the VirtQueueElement.

The advantage is that we will be able to allocate only the space that
is needed for the actual size of the s/g list instead of the full
VIRTQUEUE_MAX_SIZE items.  Currently VirtQueueElement takes about 48K
of memory, and this kind of allocation puts a lot of stress on malloc.
By cutting the size by two or three orders of magnitude, malloc can
use much more efficient algorithms.

The patch is pretty large, but changes to each device are testable
more or less independently.  Splitting it would mostly add churn.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

3 files changed, 12 insertions, 9 deletions

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 15fb0ab75d..db5f4780dc 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -1587,7 +1587,7 @@ static int v9fs_xattr_read(V9fsState *s, V9fsPDU *pdu, V9fsFidState *fidp,
     int read_count;
     int64_t xattr_len;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
-    VirtQueueElement *elem = &v->elems[pdu->idx];
+    VirtQueueElement *elem = v->elems[pdu->idx];
 
     xattr_len = fidp->fs.xattr.len;
     read_count = xattr_len - off;
diff --git a/hw/9pfs/virtio-9p-device.c b/hw/9pfs/virtio-9p-device.c
index c5f7b92640..a38850ee89 100644
--- a/hw/9pfs/virtio-9p-device.c
+++ b/hw/9pfs/virtio-9p-device.c
@@ -26,10 +26,12 @@ void virtio_9p_push_and_notify(V9fsPDU *pdu)
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
-    VirtQueueElement *elem = &v->elems[pdu->idx];
+    VirtQueueElement *elem = v->elems[pdu->idx];
 
     /* push onto queue and notify */
     virtqueue_push(v->vq, elem, pdu->size);
+    g_free(elem);
+    v->elems[pdu->idx] = NULL;
 
     /* FIXME: we should batch these completions */
     virtio_notify(VIRTIO_DEVICE(v), v->vq);
@@ -48,10 +50,10 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
             uint8_t id;
             uint16_t tag_le;
         } QEMU_PACKED out;
-        VirtQueueElement *elem = &v->elems[pdu->idx];
+        VirtQueueElement *elem;
 
-        len = virtqueue_pop(vq, elem);
-        if (!len) {
+        elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
+        if (!elem) {
             pdu_free(pdu);
             break;
         }
@@ -59,6 +61,7 @@ static void handle_9p_output(VirtIODevice *vdev, VirtQueue *vq)
         BUG_ON(elem->out_num == 0 || elem->in_num == 0);
         QEMU_BUILD_BUG_ON(sizeof out != 7);
 
+        v->elems[pdu->idx] = elem;
         len = iov_to_buf(elem->out_sg, elem->out_num, 0,
                          &out, sizeof out);
         BUG_ON(len != sizeof out);
@@ -141,7 +144,7 @@ ssize_t virtio_pdu_vmarshal(V9fsPDU *pdu, size_t offset,
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
-    VirtQueueElement *elem = &v->elems[pdu->idx];
+    VirtQueueElement *elem = v->elems[pdu->idx];
 
     return v9fs_iov_vmarshal(elem->in_sg, elem->in_num, offset, 1, fmt, ap);
 }
@@ -151,7 +154,7 @@ ssize_t virtio_pdu_vunmarshal(V9fsPDU *pdu, size_t offset,
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
-    VirtQueueElement *elem = &v->elems[pdu->idx];
+    VirtQueueElement *elem = v->elems[pdu->idx];
 
     return v9fs_iov_vunmarshal(elem->out_sg, elem->out_num, offset, 1, fmt, ap);
 }
@@ -161,7 +164,7 @@ void virtio_init_iov_from_pdu(V9fsPDU *pdu, struct iovec **piov,
 {
     V9fsState *s = pdu->s;
     V9fsVirtioState *v = container_of(s, V9fsVirtioState, state);
-    VirtQueueElement *elem = &v->elems[pdu->idx];
+    VirtQueueElement *elem = v->elems[pdu->idx];
 
     if (is_write) {
         *piov = elem->out_sg;
diff --git a/hw/9pfs/virtio-9p.h b/hw/9pfs/virtio-9p.h
index 1cdf0a2d65..7f6d885539 100644
--- a/hw/9pfs/virtio-9p.h
+++ b/hw/9pfs/virtio-9p.h
@@ -11,7 +11,7 @@ typedef struct V9fsVirtioState
     VirtQueue *vq;
     size_t config_size;
     V9fsPDU pdus[MAX_REQ];
-    VirtQueueElement elems[MAX_REQ];
+    VirtQueueElement *elems[MAX_REQ];
     V9fsState state;
 } V9fsVirtioState;