[Qemu-devel] [PATCH 04/14] virtio-net: avoid sg copy

["Michael S. Tsirkin" <mst@redhat.com>]

Avoid tweaking iovec during receive. This removes
the need to copy the vector.
Note: we already have an evil cast in work_around_broken_dhclient
and unfortunately this adds another one.
const on buf is ignored by this function anyway so arguably
this is not making things much worse.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 hw/virtio-net.c | 45 +++++++++++++++++++++------------------------
 iov.h           |  8 ++++----
 2 files changed, 25 insertions(+), 28 deletions(-)

diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 0c5081e..6e53858 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -503,7 +503,7 @@ static int virtio_net_has_buffers(VirtIONet *n, int bufsize)
  * we should provide a mechanism to disable it to avoid polluting the host
  * cache.
  */
-static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
+static void work_around_broken_dhclient(const struct virtio_net_hdr *hdr,
                                         const uint8_t *buf, size_t size)
 {
     if ((hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && /* missing csum */
@@ -513,31 +513,27 @@ static void work_around_broken_dhclient(struct virtio_net_hdr *hdr,
         (buf[34] == 0 && buf[35] == 67)) { /* udp.srcport == bootps */
         /* FIXME this cast is evil */
         net_checksum_calculate((uint8_t *)buf, size);
-        hdr->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
+        ((struct virtio_net_hdr *)hdr)->flags &= ~VIRTIO_NET_HDR_F_NEEDS_CSUM;
     }
 }
 
-static int receive_header(VirtIONet *n, struct iovec *iov, int iovcnt,
-                          const void *buf, size_t size, size_t hdr_len)
+static int receive_header(VirtIONet *n, const struct iovec *iov, int iov_cnt,
+                          const void *buf, size_t size)
 {
-    struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)iov[0].iov_base;
     int offset = 0;
 
-    hdr->flags = 0;
-    hdr->gso_type = VIRTIO_NET_HDR_GSO_NONE;
-
     if (n->has_vnet_hdr) {
-        memcpy(hdr, buf, sizeof(*hdr));
-        offset = sizeof(*hdr);
-        work_around_broken_dhclient(hdr, buf + offset, size - offset);
+        work_around_broken_dhclient(buf, buf + offset, size - offset);
+        offset = sizeof(struct virtio_net_hdr);
+        iov_from_buf(iov, iov_cnt, 0, buf, offset);
+    } else {
+        struct virtio_net_hdr hdr = {
+            .flags = 0,
+            .gso_type = VIRTIO_NET_HDR_GSO_NONE
+        };
+        iov_from_buf(iov, iov_cnt, 0, &hdr, sizeof hdr);
     }
 
-    /* We only ever receive a struct virtio_net_hdr from the tapfd,
-     * but we may be passing along a larger header to the guest.
-     */
-    iov[0].iov_base += hdr_len;
-    iov[0].iov_len  -= hdr_len;
-
     return offset;
 }
 
@@ -598,7 +594,8 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
 {
     VirtIONet *n = DO_UPCAST(NICState, nc, nc)->opaque;
     struct virtio_net_hdr_mrg_rxbuf *mhdr = NULL;
-    size_t offset, i;
+    const struct iovec *sg = elem.in_sg;
+    size_t offset, i, guest_offset;
 
     if (!virtio_net_can_receive(&n->nic->nc))
         return -1;
@@ -615,7 +612,7 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
     while (offset < size) {
         VirtQueueElement elem;
         int len, total;
-        struct iovec sg[VIRTQUEUE_MAX_SIZE];
+        const struct iovec *sg = elem.in_sg;
 
         total = 0;
 
@@ -640,20 +637,20 @@ static ssize_t virtio_net_receive(NetClientState *nc, const uint8_t *buf, size_t
             exit(1);
         }
 
-        memcpy(&sg, &elem.in_sg[0], sizeof(sg[0]) * elem.in_num);
-
         if (i == 0) {
             if (n->mergeable_rx_bufs)
                 mhdr = (struct virtio_net_hdr_mrg_rxbuf *)sg[0].iov_base;
 
             offset += receive_header(n, sg, elem.in_num,
-                                     buf + offset, size - offset,
-                                     n->guest_hdr_len);
+                                     buf + offset, size - offset);
             total += n->guest_hdr_len;
+            guest_offset = n->guest_hdr_len;
+        } else {
+            guest_offset = 0;
         }
 
         /* copy in packet.  ugh */
-        len = iov_from_buf(sg, elem.in_num, 0,
+        len = iov_from_buf(sg, elem.in_num, guest_offset,
                            buf + offset, size - offset);
         total += len;
         offset += len;
diff --git a/iov.h b/iov.h
index f0daefa..6c9fdf1 100644
--- a/iov.h
+++ b/iov.h
@@ -90,8 +90,8 @@ void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
 /*
  * Partial copy of vector from iov to dst_iov (data is not copied).
  * dst_iov overlaps iov at a specified offset.
- * size of dst_iov is at most bytes. Actual size is returned.
+ * size of dst_iov is at most bytes. dst vector count is returned.
  */
-size_t iov_cpy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
-               const struct iovec *iov, unsigned int iov_cnt,
-               size_t offset, size_t bytes);
+unsigned iov_cpy(struct iovec *dst_iov, unsigned int dst_iov_cnt,
+                 const struct iovec *iov, unsigned int iov_cnt,
+                 size_t offset, size_t bytes);
-- 
MST