The upcoming 'fixed-ram' feature would require qemu to write data at
specific offsets of the file. This is currently missing so this patch
adds it. I've chosen to implement it as a type-specific function rather
than plumb it through the generic channel interface as it relies on
being able to do seeks.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 include/io/channel-file.h |  5 +++++
 io/channel-file.c         | 24 ++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/include/io/channel-file.h b/include/io/channel-file.h
index 50e8eb113868..0a5d54f5e58e 100644
--- a/include/io/channel-file.h
+++ b/include/io/channel-file.h
@@ -89,4 +89,9 @@ qio_channel_file_new_path(const char *path,
                           mode_t mode,
                           Error **errp);
 
+ssize_t qio_channel_file_pwritev(QIOChannel *ioc,
+				 const struct iovec *iov,
+				 size_t niov,
+				 off_t offset,
+				 Error **errp);
 #endif /* QIO_CHANNEL_FILE_H */
diff --git a/io/channel-file.c b/io/channel-file.c
index b67687c2aa64..da17d0a11ba7 100644
--- a/io/channel-file.c
+++ b/io/channel-file.c
@@ -136,6 +136,30 @@ static ssize_t qio_channel_file_writev(QIOChannel *ioc,
     return ret;
 }
 
+ssize_t qio_channel_file_pwritev(QIOChannel *ioc,
+				 const struct iovec *iov,
+				 size_t niov,
+				 off_t offset,
+				 Error **errp)
+{
+    QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
+    ssize_t ret;
+
+ retry:
+    ret = pwritev(fioc->fd, iov, niov, offset);
+    if (ret <= 0) {
+        if (errno == EAGAIN) {
+            return QIO_CHANNEL_ERR_BLOCK;
+        }
+        if (errno == EINTR) {
+            goto retry;
+        }
+        error_setg_errno(errp, errno, "Unable to write to file");
+        return -1;
+    }
+    return ret;
+}
+
 static int qio_channel_file_set_blocking(QIOChannel *ioc,
                                          bool enabled,
                                          Error **errp)
-- 
2.34.1