Add qemu_get_buffer_in_place to avoid copies some of the time

qemu_get_buffer always copies the data it reads to a users buffer,
however in many cases the file buffer inside qemu_file could be given
back to the caller, avoiding the copy.  This isn't always possible
depending on the size and alignment of the data.

Thus 'qemu_get_buffer_in_place' either copies the data to a supplied
buffer or updates a pointer to the internal buffer if convenient.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/qemu-file.h

@@ -163,9 +163,11 @@ void qemu_put_be32(QEMUFile *f, unsigned int v);
 void qemu_put_be64(QEMUFile *f, uint64_t v);
 size_t qemu_peek_buffer(QEMUFile *f, uint8_t **buf, size_t size, size_t offset);
 size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, size_t size);
+size_t qemu_get_buffer_in_place(QEMUFile *f, uint8_t **buf, size_t size);
 ssize_t qemu_put_compression_data(QEMUFile *f, const uint8_t *p, size_t size,
                                   int level);
 int qemu_put_qemu_file(QEMUFile *f_des, QEMUFile *f_src);
+
 /*
  * Note that you can only peek continuous bytes from where the current pointer
  * is; you aren't guaranteed to be able to peak to +n bytes unless you've

migration/qemu-file.c

@@ -433,6 +433,43 @@ size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, size_t size)
     return done;
 }
 
+/*
+ * Read 'size' bytes of data from the file.
+ * 'size' can be larger than the internal buffer.
+ *
+ * The data:
+ *   may be held on an internal buffer (in which case *buf is updated
+ *     to point to it) that is valid until the next qemu_file operation.
+ * OR
+ *   will be copied to the *buf that was passed in.
+ *
+ * The code tries to avoid the copy if possible.
+ *
+ * It will return size bytes unless there was an error, in which case it will
+ * return as many as it managed to read (assuming blocking fd's which
+ * all current QEMUFile are)
+ *
+ * Note: Since **buf may get changed, the caller should take care to
+ *       keep a pointer to the original buffer if it needs to deallocate it.
+ */
+size_t qemu_get_buffer_in_place(QEMUFile *f, uint8_t **buf, size_t size)
+{
+    if (size < IO_BUF_SIZE) {
+        size_t res;
+        uint8_t *src;
+
+        res = qemu_peek_buffer(f, &src, size, 0);
+
+        if (res == size) {
+            qemu_file_skip(f, res);
+            *buf = src;
+            return res;
+        }
+    }
+
+    return qemu_get_buffer(f, *buf, size);
+}
+
 /*
  * Peeks a single byte from the buffer; this isn't guaranteed to work if
  * offset leaves a gap after the previous read/peeked data.