change iov_* function prototypes to be more appropriate

Reorder arguments to be more natural, readable and
consistent with other iov_* functions, and change
argument names, from:
 iov_from_buf(iov, iov_cnt, buf, iov_off, size)
to
 iov_from_buf(iov, iov_cnt, offset, buf, bytes)

The result becomes natural English:

 copy data to this `iov' vector with `iov_cnt'
 elements starting at byte offset `offset'
 from memory buffer `buf', processing `bytes'
 bytes max.

(Try to read the original prototype this way).

Also change iov_clear() to more general iov_memset()
(it uses memset() internally anyway).

While at it, add comments to the header file
describing what the routines actually does.

The patch only renames argumens in the header, but
keeps old names in the implementation.  The next
patch will touch actual code to match.

Now, it might look wrong to pay so much attention
to so small things.  But we've so many badly designed
interfaces already so the whole thing becomes rather
confusing or error prone.  One example of this is
previous commit and small discussion which emerged
from it, with an outcome that the utility functions
like these aren't well-understdandable, leading to
strange usage cases.  That's why I paid quite some
attention to this set of functions and a few
others in subsequent patches.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/rtl8139.c

@@ -1783,7 +1783,7 @@ static void rtl8139_transfer_frame(RTL8139State *s, uint8_t *buf, int size,
         if (iov) {
             buf2_size = iov_size(iov, 3);
             buf2 = g_malloc(buf2_size);
-            iov_to_buf(iov, 3, buf2, 0, buf2_size);
+            iov_to_buf(iov, 3, 0, buf2, buf2_size);
             buf = buf2;
         }
 

hw/usb/core.c

@@ -522,10 +522,10 @@ void usb_packet_copy(USBPacket *p, void *ptr, size_t bytes)
     switch (p->pid) {
     case USB_TOKEN_SETUP:
     case USB_TOKEN_OUT:
-        iov_to_buf(p->iov.iov, p->iov.niov, ptr, p->result, bytes);
+        iov_to_buf(p->iov.iov, p->iov.niov, p->result, ptr, bytes);
         break;
     case USB_TOKEN_IN:
-        iov_from_buf(p->iov.iov, p->iov.niov, ptr, p->result, bytes);
+        iov_from_buf(p->iov.iov, p->iov.niov, p->result, ptr, bytes);
         break;
     default:
         fprintf(stderr, "%s: invalid pid: %x\n", __func__, p->pid);
@@ -539,7 +539,7 @@ void usb_packet_skip(USBPacket *p, size_t bytes)
     assert(p->result >= 0);
     assert(p->result + bytes <= p->iov.size);
     if (p->pid == USB_TOKEN_IN) {
-        iov_clear(p->iov.iov, p->iov.niov, p->result, bytes);
+        iov_memset(p->iov.iov, p->iov.niov, p->result, 0, bytes);
     }
     p->result += bytes;
 }

hw/virtio-balloon.c

@@ -77,7 +77,7 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
         size_t offset = 0;
         uint32_t pfn;
 
-        while (iov_to_buf(elem.out_sg, elem.out_num, &pfn, offset, 4) == 4) {
+        while (iov_to_buf(elem.out_sg, elem.out_num, offset, &pfn, 4) == 4) {
             ram_addr_t pa;
             ram_addr_t addr;
 
@@ -118,7 +118,7 @@ static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
      */
     reset_stats(s);
 
-    while (iov_to_buf(elem->out_sg, elem->out_num, &stat, offset, sizeof(stat))
+    while (iov_to_buf(elem->out_sg, elem->out_num, offset, &stat, sizeof(stat))
            == sizeof(stat)) {
         uint16_t tag = tswap16(stat.tag);
         uint64_t val = tswap64(stat.val);

hw/virtio-net.c

@@ -656,8 +656,8 @@ static ssize_t virtio_net_receive(VLANClientState *nc, const uint8_t *buf, size_
         }
 
         /* copy in packet.  ugh */
-        len = iov_from_buf(sg, elem.in_num,
-                           buf + offset, 0, size - offset);
+        len = iov_from_buf(sg, elem.in_num, 0,
+                           buf + offset, size - offset);
         total += len;
         offset += len;
         /* If buffers can't be merged, at this point we

hw/virtio-serial-bus.c

@@ -106,8 +106,8 @@ static size_t write_to_port(VirtIOSerialPort *port,
             break;
         }
 
-        len = iov_from_buf(elem.in_sg, elem.in_num,
-                           buf + offset, 0, size - offset);
+        len = iov_from_buf(elem.in_sg, elem.in_num, 0,
+                           buf + offset, size - offset);
         offset += len;
 
         virtqueue_push(vq, &elem, len);
@@ -467,7 +467,7 @@ static void control_out(VirtIODevice *vdev, VirtQueue *vq)
             buf = g_malloc(cur_len);
             len = cur_len;
         }
-        iov_to_buf(elem.out_sg, elem.out_num, buf, 0, cur_len);
+        iov_to_buf(elem.out_sg, elem.out_num, 0, buf, cur_len);
 
         handle_control_message(vser, buf, cur_len);
         virtqueue_push(vq, &elem, 0);

iov.c

@@ -17,8 +17,8 @@
 
 #include "iov.h"
 
-size_t iov_from_buf(struct iovec *iov, unsigned int iov_cnt,
-                    const void *buf, size_t iov_off, size_t size)
+size_t iov_from_buf(struct iovec *iov, unsigned int iov_cnt, size_t iov_off,
+                    const void *buf, size_t size)
 {
     size_t iovec_off, buf_off;
     unsigned int i;
@@ -40,8 +40,8 @@ size_t iov_from_buf(struct iovec *iov, unsigned int iov_cnt,
     return buf_off;
 }
 
-size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
-                  void *buf, size_t iov_off, size_t size)
+size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt, size_t iov_off,
+                  void *buf, size_t size)
 {
     uint8_t *ptr;
     size_t iovec_off, buf_off;
@@ -65,8 +65,8 @@ size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
     return buf_off;
 }
 
-size_t iov_clear(const struct iovec *iov, const unsigned int iov_cnt,
-                 size_t iov_off, size_t size)
+size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
+                 size_t iov_off, int fillc, size_t size)
 {
     size_t iovec_off, buf_off;
     unsigned int i;
@@ -77,7 +77,7 @@ size_t iov_clear(const struct iovec *iov, const unsigned int iov_cnt,
         if (iov_off < (iovec_off + iov[i].iov_len)) {
             size_t len = MIN((iovec_off + iov[i].iov_len) - iov_off , size);
 
-            memset(iov[i].iov_base + (iov_off - iovec_off), 0, len);
+            memset(iov[i].iov_base + (iov_off - iovec_off), fillc, len);
 
             buf_off += len;
             iov_off += len;

iov.h

@@ -12,12 +12,44 @@
 
 #include "qemu-common.h"
 
+/**
+ * count and return data size, in bytes, of an iovec
+ * starting at `iov' of `iov_cnt' number of elements.
+ */
+size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt);
+
+/**
+ * Copy from single continuous buffer to scatter-gather vector of buffers
+ * (iovec) and back like memcpy() between two continuous memory regions.
+ * Data in single continuous buffer starting at address `buf' and
+ * `bytes' bytes long will be copied to/from an iovec `iov' with
+ * `iov_cnt' number of elements, starting at byte position `offset'
+ * within the iovec.  If the iovec does not contain enough space,
+ * only part of data will be copied, up to the end of the iovec.
+ * Number of bytes actually copied will be returned, which is
+ *  min(bytes, iov_size(iov)-offset)
+ */
 size_t iov_from_buf(struct iovec *iov, unsigned int iov_cnt,
-                    const void *buf, size_t iov_off, size_t size);
+                    size_t offset, const void *buf, size_t bytes);
 size_t iov_to_buf(const struct iovec *iov, const unsigned int iov_cnt,
-                  void *buf, size_t iov_off, size_t size);
-size_t iov_size(const struct iovec *iov, const unsigned int iov_cnt);
-size_t iov_clear(const struct iovec *iov, const unsigned int iov_cnt,
-                 size_t iov_off, size_t size);
+                  size_t offset, void *buf, size_t bytes);
+
+/**
+ * Set data bytes pointed out by iovec `iov' of size `iov_cnt' elements,
+ * starting at byte offset `start', to value `fillc', repeating it
+ * `bytes' number of times.
+ * If `bytes' is large enough, only last bytes portion of iovec,
+ * up to the end of it, will be filled with the specified value.
+ * Function return actual number of bytes processed, which is
+ * min(size, iov_size(iov) - offset).
+ */
+size_t iov_memset(const struct iovec *iov, const unsigned int iov_cnt,
+                  size_t offset, int fillc, size_t bytes);
+
+/**
+ * Produce a text hexdump of iovec `iov' with `iov_cnt' number of elements
+ * in file `fp', prefixing each line with `prefix' and processing not more
+ * than `limit' data bytes.
+ */
 void iov_hexdump(const struct iovec *iov, const unsigned int iov_cnt,
                  FILE *fp, const char *prefix, size_t limit);

net.c

@@ -544,7 +544,7 @@ static ssize_t vc_sendv_compat(VLANClientState *vc, const struct iovec *iov,
     uint8_t buffer[4096];
     size_t offset;
 
-    offset = iov_to_buf(iov, iovcnt, buffer, 0, sizeof(buffer));
+    offset = iov_to_buf(iov, iovcnt, 0, buffer, sizeof(buffer));
 
     return vc->info->receive(vc, buffer, offset);
 }