diff mbox series

[v8,1/5] QIOChannel: Add flags on io_writev and introduce io_flush callback

Message ID 20220201062901.428838-2-leobras@redhat.com (mailing list archive)
State New, archived
Headers show
Series MSG_ZEROCOPY + multifd | expand

Commit Message

Leonardo Bras Feb. 1, 2022, 6:28 a.m. UTC
Add flags to io_writev and introduce io_flush as optional callback to
QIOChannelClass, allowing the implementation of zero copy writes by
subclasses.

How to use them:
- Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
- Wait write completion with qio_channel_flush().

Notes:
As some zero copy write implementations work asynchronously, it's
recommended to keep the write buffer untouched until the return of
qio_channel_flush(), to avoid the risk of sending an updated buffer
instead of the buffer state during write.

As io_flush callback is optional, if a subclass does not implement it, then:
- io_flush will return 0 without changing anything.

Also, some functions like qio_channel_writev_full_all() were adapted to
receive a flag parameter. That allows shared code between zero copy and
non-zero copy writev, and also an easier implementation on new flags.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
 include/io/channel.h                | 38 ++++++++++++++++++++-
 chardev/char-io.c                   |  2 +-
 hw/remote/mpqemu-link.c             |  2 +-
 io/channel-buffer.c                 |  1 +
 io/channel-command.c                |  1 +
 io/channel-file.c                   |  1 +
 io/channel-socket.c                 |  2 ++
 io/channel-tls.c                    |  1 +
 io/channel-websock.c                |  1 +
 io/channel.c                        | 53 +++++++++++++++++++++++------
 migration/rdma.c                    |  1 +
 scsi/pr-manager-helper.c            |  2 +-
 tests/unit/test-io-channel-socket.c |  1 +
 13 files changed, 92 insertions(+), 14 deletions(-)

Comments

Daniel P. Berrangé Feb. 1, 2022, 9:35 a.m. UTC | #1
On Tue, Feb 01, 2022 at 03:28:59AM -0300, Leonardo Bras wrote:
> Add flags to io_writev and introduce io_flush as optional callback to
> QIOChannelClass, allowing the implementation of zero copy writes by
> subclasses.
> 
> How to use them:
> - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> - Wait write completion with qio_channel_flush().
> 
> Notes:
> As some zero copy write implementations work asynchronously, it's
> recommended to keep the write buffer untouched until the return of
> qio_channel_flush(), to avoid the risk of sending an updated buffer
> instead of the buffer state during write.
> 
> As io_flush callback is optional, if a subclass does not implement it, then:
> - io_flush will return 0 without changing anything.
> 
> Also, some functions like qio_channel_writev_full_all() were adapted to
> receive a flag parameter. That allows shared code between zero copy and
> non-zero copy writev, and also an easier implementation on new flags.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  include/io/channel.h                | 38 ++++++++++++++++++++-
>  chardev/char-io.c                   |  2 +-
>  hw/remote/mpqemu-link.c             |  2 +-
>  io/channel-buffer.c                 |  1 +
>  io/channel-command.c                |  1 +
>  io/channel-file.c                   |  1 +
>  io/channel-socket.c                 |  2 ++
>  io/channel-tls.c                    |  1 +
>  io/channel-websock.c                |  1 +
>  io/channel.c                        | 53 +++++++++++++++++++++++------
>  migration/rdma.c                    |  1 +
>  scsi/pr-manager-helper.c            |  2 +-
>  tests/unit/test-io-channel-socket.c |  1 +
>  13 files changed, 92 insertions(+), 14 deletions(-)
> 
> diff --git a/io/channel.c b/io/channel.c
> index e8b019dc36..b8b99fdc4c 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -72,18 +72,32 @@ ssize_t qio_channel_writev_full(QIOChannel *ioc,
>                                  size_t niov,
>                                  int *fds,
>                                  size_t nfds,
> +                                int flags,
>                                  Error **errp)
>  {
>      QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
>  
> -    if ((fds || nfds) &&
> -        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> +    if (fds || nfds) {
> +        if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> +            error_setg_errno(errp, EINVAL,
> +                             "Channel does not support file descriptor passing");
> +            return -1;
> +        }
> +        if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> +            error_setg_errno(errp, EINVAL,
> +                             "Zero Copy does not support file descriptor passing");
> +            return -1;
> +        }

Here you gracefully reject FD passing when zero copy is requested
which is good.

> +    }
> +

> @@ -235,10 +249,16 @@ int qio_channel_writev_full_all(QIOChannel *ioc,
>                            iov, niov,
>                            0, iov_size(iov, niov));
>  
> +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> +        assert(fds == NULL && nfds == 0);
> +    }

But here you  abort QEMU if FD passing is requested when zero copy
is set.

AFAICT, if you just delete this assert, the code to gracefully
report errors will do the right thing.

Without the assert:

  Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Regards,
Daniel
Leonardo Bras Feb. 1, 2022, 5:25 p.m. UTC | #2
Hello Daniel, thanks for reviewing!

On Tue, Feb 1, 2022 at 6:35 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Tue, Feb 01, 2022 at 03:28:59AM -0300, Leonardo Bras wrote:
> > Add flags to io_writev and introduce io_flush as optional callback to
> > QIOChannelClass, allowing the implementation of zero copy writes by
> > subclasses.
> >
> > How to use them:
> > - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> > - Wait write completion with qio_channel_flush().
> >
> > Notes:
> > As some zero copy write implementations work asynchronously, it's
> > recommended to keep the write buffer untouched until the return of
> > qio_channel_flush(), to avoid the risk of sending an updated buffer
> > instead of the buffer state during write.
> >
> > As io_flush callback is optional, if a subclass does not implement it, then:
> > - io_flush will return 0 without changing anything.
> >
> > Also, some functions like qio_channel_writev_full_all() were adapted to
> > receive a flag parameter. That allows shared code between zero copy and
> > non-zero copy writev, and also an easier implementation on new flags.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  include/io/channel.h                | 38 ++++++++++++++++++++-
> >  chardev/char-io.c                   |  2 +-
> >  hw/remote/mpqemu-link.c             |  2 +-
> >  io/channel-buffer.c                 |  1 +
> >  io/channel-command.c                |  1 +
> >  io/channel-file.c                   |  1 +
> >  io/channel-socket.c                 |  2 ++
> >  io/channel-tls.c                    |  1 +
> >  io/channel-websock.c                |  1 +
> >  io/channel.c                        | 53 +++++++++++++++++++++++------
> >  migration/rdma.c                    |  1 +
> >  scsi/pr-manager-helper.c            |  2 +-
> >  tests/unit/test-io-channel-socket.c |  1 +
> >  13 files changed, 92 insertions(+), 14 deletions(-)
> >
> > diff --git a/io/channel.c b/io/channel.c
> > index e8b019dc36..b8b99fdc4c 100644
> > --- a/io/channel.c
> > +++ b/io/channel.c
> > @@ -72,18 +72,32 @@ ssize_t qio_channel_writev_full(QIOChannel *ioc,
> >                                  size_t niov,
> >                                  int *fds,
> >                                  size_t nfds,
> > +                                int flags,
> >                                  Error **errp)
> >  {
> >      QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
> >
> > -    if ((fds || nfds) &&
> > -        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> > +    if (fds || nfds) {
> > +        if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
> > +            error_setg_errno(errp, EINVAL,
> > +                             "Channel does not support file descriptor passing");
> > +            return -1;
> > +        }
> > +        if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> > +            error_setg_errno(errp, EINVAL,
> > +                             "Zero Copy does not support file descriptor passing");
> > +            return -1;
> > +        }
>
> Here you gracefully reject FD passing when zero copy is requested
> which is good.
>
> > +    }
> > +
>
> > @@ -235,10 +249,16 @@ int qio_channel_writev_full_all(QIOChannel *ioc,
> >                            iov, niov,
> >                            0, iov_size(iov, niov));
> >
> > +    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
> > +        assert(fds == NULL && nfds == 0);
> > +    }
>
> But here you  abort QEMU if FD passing is requested when zero copy
> is set.
>
> AFAICT, if you just delete this assert, the code to gracefully
> report errors will do the right thing.

Yeah, thatś right. This test is unnecessary since qio_channel_writev_full()
will be called and will return error if fds + zerocopy happens.

Good catch!

>
> Without the assert:
>
>   Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>

Thanks!
I will wait for more feedback on other patches before sending the v9,
but it should not take too long this time.

Best regards,
Leo
Peter Xu Feb. 7, 2022, 12:49 p.m. UTC | #3
On Tue, Feb 01, 2022 at 03:28:59AM -0300, Leonardo Bras wrote:
> Add flags to io_writev and introduce io_flush as optional callback to
> QIOChannelClass, allowing the implementation of zero copy writes by
> subclasses.
> 
> How to use them:
> - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> - Wait write completion with qio_channel_flush().
> 
> Notes:
> As some zero copy write implementations work asynchronously, it's
> recommended to keep the write buffer untouched until the return of
> qio_channel_flush(), to avoid the risk of sending an updated buffer
> instead of the buffer state during write.
> 
> As io_flush callback is optional, if a subclass does not implement it, then:
> - io_flush will return 0 without changing anything.
> 
> Also, some functions like qio_channel_writev_full_all() were adapted to
> receive a flag parameter. That allows shared code between zero copy and
> non-zero copy writev, and also an easier implementation on new flags.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>

With Dan's comment addressed on removing the redundant assertion:

Reviewed-by: Peter Xu <peterx@redhat.com>
Leonardo Bras Feb. 7, 2022, 8:50 p.m. UTC | #4
Hello Peter,

On Mon, Feb 7, 2022 at 9:50 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Feb 01, 2022 at 03:28:59AM -0300, Leonardo Bras wrote:
> > Add flags to io_writev and introduce io_flush as optional callback to
> > QIOChannelClass, allowing the implementation of zero copy writes by
> > subclasses.
> >
> > How to use them:
> > - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> > - Wait write completion with qio_channel_flush().
> >
> > Notes:
> > As some zero copy write implementations work asynchronously, it's
> > recommended to keep the write buffer untouched until the return of
> > qio_channel_flush(), to avoid the risk of sending an updated buffer
> > instead of the buffer state during write.
> >
> > As io_flush callback is optional, if a subclass does not implement it, then:
> > - io_flush will return 0 without changing anything.
> >
> > Also, some functions like qio_channel_writev_full_all() were adapted to
> > receive a flag parameter. That allows shared code between zero copy and
> > non-zero copy writev, and also an easier implementation on new flags.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
>
> With Dan's comment addressed on removing the redundant assertion:
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>

Thank you for reviewing!

I think I am now missing reviewing only on patch 5/5 before sending
the next version.
Could you and/or Daniel help me with that? Just to check if I am
missing anything?

Best regards,
Leo
Juan Quintela Feb. 18, 2022, 4:36 p.m. UTC | #5
Leonardo Bras <leobras@redhat.com> wrote:
> Add flags to io_writev and introduce io_flush as optional callback to
> QIOChannelClass, allowing the implementation of zero copy writes by
> subclasses.
>
> How to use them:
> - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> - Wait write completion with qio_channel_flush().
>
> Notes:
> As some zero copy write implementations work asynchronously, it's
> recommended to keep the write buffer untouched until the return of
> qio_channel_flush(), to avoid the risk of sending an updated buffer
> instead of the buffer state during write.
>
> As io_flush callback is optional, if a subclass does not implement it, then:
> - io_flush will return 0 without changing anything.
>
> Also, some functions like qio_channel_writev_full_all() were adapted to
> receive a flag parameter. That allows shared code between zero copy and
> non-zero copy writev, and also an easier implementation on new flags.
>
> Signed-off-by: Leonardo Bras <leobras@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

As everybody pointed out about the missing assertion...
Leonardo Bras Feb. 21, 2022, 4:41 p.m. UTC | #6
Thanks for reviewing, Juan!

On Fri, Feb 18, 2022 at 1:36 PM Juan Quintela <quintela@redhat.com> wrote:
>
> Leonardo Bras <leobras@redhat.com> wrote:
> > Add flags to io_writev and introduce io_flush as optional callback to
> > QIOChannelClass, allowing the implementation of zero copy writes by
> > subclasses.
> >
> > How to use them:
> > - Write data using qio_channel_writev*(...,QIO_CHANNEL_WRITE_FLAG_ZERO_COPY),
> > - Wait write completion with qio_channel_flush().
> >
> > Notes:
> > As some zero copy write implementations work asynchronously, it's
> > recommended to keep the write buffer untouched until the return of
> > qio_channel_flush(), to avoid the risk of sending an updated buffer
> > instead of the buffer state during write.
> >
> > As io_flush callback is optional, if a subclass does not implement it, then:
> > - io_flush will return 0 without changing anything.
> >
> > Also, some functions like qio_channel_writev_full_all() were adapted to
> > receive a flag parameter. That allows shared code between zero copy and
> > non-zero copy writev, and also an easier implementation on new flags.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
>
> As everybody pointed out about the missing assertion...
>
diff mbox series

Patch

diff --git a/include/io/channel.h b/include/io/channel.h
index 88988979f8..c680ee7480 100644
--- a/include/io/channel.h
+++ b/include/io/channel.h
@@ -32,12 +32,15 @@  OBJECT_DECLARE_TYPE(QIOChannel, QIOChannelClass,
 
 #define QIO_CHANNEL_ERR_BLOCK -2
 
+#define QIO_CHANNEL_WRITE_FLAG_ZERO_COPY 0x1
+
 typedef enum QIOChannelFeature QIOChannelFeature;
 
 enum QIOChannelFeature {
     QIO_CHANNEL_FEATURE_FD_PASS,
     QIO_CHANNEL_FEATURE_SHUTDOWN,
     QIO_CHANNEL_FEATURE_LISTEN,
+    QIO_CHANNEL_FEATURE_WRITE_ZERO_COPY,
 };
 
 
@@ -104,6 +107,7 @@  struct QIOChannelClass {
                          size_t niov,
                          int *fds,
                          size_t nfds,
+                         int flags,
                          Error **errp);
     ssize_t (*io_readv)(QIOChannel *ioc,
                         const struct iovec *iov,
@@ -136,6 +140,8 @@  struct QIOChannelClass {
                                   IOHandler *io_read,
                                   IOHandler *io_write,
                                   void *opaque);
+    int (*io_flush)(QIOChannel *ioc,
+                    Error **errp);
 };
 
 /* General I/O handling functions */
@@ -228,6 +234,7 @@  ssize_t qio_channel_readv_full(QIOChannel *ioc,
  * @niov: the length of the @iov array
  * @fds: an array of file handles to send
  * @nfds: number of file handles in @fds
+ * @flags: write flags (QIO_CHANNEL_WRITE_FLAG_*)
  * @errp: pointer to a NULL-initialized error object
  *
  * Write data to the IO channel, reading it from the
@@ -260,6 +267,7 @@  ssize_t qio_channel_writev_full(QIOChannel *ioc,
                                 size_t niov,
                                 int *fds,
                                 size_t nfds,
+                                int flags,
                                 Error **errp);
 
 /**
@@ -837,6 +845,7 @@  int qio_channel_readv_full_all(QIOChannel *ioc,
  * @niov: the length of the @iov array
  * @fds: an array of file handles to send
  * @nfds: number of file handles in @fds
+ * @flags: write flags (QIO_CHANNEL_WRITE_FLAG_*)
  * @errp: pointer to a NULL-initialized error object
  *
  *
@@ -846,6 +855,14 @@  int qio_channel_readv_full_all(QIOChannel *ioc,
  * to be written, yielding from the current coroutine
  * if required.
  *
+ * If QIO_CHANNEL_WRITE_FLAG_ZERO_COPY is passed in flags,
+ * instead of waiting for all requested data to be written,
+ * this function will wait until it's all queued for writing.
+ * In this case, if the buffer gets changed between queueing and
+ * sending, the updated buffer will be sent. If this is not a
+ * desired behavior, it's suggested to call qio_channel_flush()
+ * before reusing the buffer.
+ *
  * Returns: 0 if all bytes were written, or -1 on error
  */
 
@@ -853,6 +870,25 @@  int qio_channel_writev_full_all(QIOChannel *ioc,
                                 const struct iovec *iov,
                                 size_t niov,
                                 int *fds, size_t nfds,
-                                Error **errp);
+                                int flags, Error **errp);
+
+/**
+ * qio_channel_flush:
+ * @ioc: the channel object
+ * @errp: pointer to a NULL-initialized error object
+ *
+ * Will block until every packet queued with
+ * qio_channel_writev_full() + QIO_CHANNEL_WRITE_FLAG_ZERO_COPY
+ * is sent, or return in case of any error.
+ *
+ * If not implemented, acts as a no-op, and returns 0.
+ *
+ * Returns -1 if any error is found,
+ *          1 if every send failed to use zero copy.
+ *          0 otherwise.
+ */
+
+int qio_channel_flush(QIOChannel *ioc,
+                      Error **errp);
 
 #endif /* QIO_CHANNEL_H */
diff --git a/chardev/char-io.c b/chardev/char-io.c
index 8ced184160..4451128cba 100644
--- a/chardev/char-io.c
+++ b/chardev/char-io.c
@@ -122,7 +122,7 @@  int io_channel_send_full(QIOChannel *ioc,
 
         ret = qio_channel_writev_full(
             ioc, &iov, 1,
-            fds, nfds, NULL);
+            fds, nfds, 0, NULL);
         if (ret == QIO_CHANNEL_ERR_BLOCK) {
             if (offset) {
                 return offset;
diff --git a/hw/remote/mpqemu-link.c b/hw/remote/mpqemu-link.c
index 7e841820e5..e8f556bd27 100644
--- a/hw/remote/mpqemu-link.c
+++ b/hw/remote/mpqemu-link.c
@@ -69,7 +69,7 @@  bool mpqemu_msg_send(MPQemuMsg *msg, QIOChannel *ioc, Error **errp)
     }
 
     if (!qio_channel_writev_full_all(ioc, send, G_N_ELEMENTS(send),
-                                    fds, nfds, errp)) {
+                                    fds, nfds, 0, errp)) {
         ret = true;
     } else {
         trace_mpqemu_send_io_error(msg->cmd, msg->size, nfds);
diff --git a/io/channel-buffer.c b/io/channel-buffer.c
index baa4e2b089..bf52011be2 100644
--- a/io/channel-buffer.c
+++ b/io/channel-buffer.c
@@ -81,6 +81,7 @@  static ssize_t qio_channel_buffer_writev(QIOChannel *ioc,
                                          size_t niov,
                                          int *fds,
                                          size_t nfds,
+                                         int flags,
                                          Error **errp)
 {
     QIOChannelBuffer *bioc = QIO_CHANNEL_BUFFER(ioc);
diff --git a/io/channel-command.c b/io/channel-command.c
index 338da73ade..54560464ae 100644
--- a/io/channel-command.c
+++ b/io/channel-command.c
@@ -258,6 +258,7 @@  static ssize_t qio_channel_command_writev(QIOChannel *ioc,
                                           size_t niov,
                                           int *fds,
                                           size_t nfds,
+                                          int flags,
                                           Error **errp)
 {
     QIOChannelCommand *cioc = QIO_CHANNEL_COMMAND(ioc);
diff --git a/io/channel-file.c b/io/channel-file.c
index d7cf6d278f..ef6807a6be 100644
--- a/io/channel-file.c
+++ b/io/channel-file.c
@@ -114,6 +114,7 @@  static ssize_t qio_channel_file_writev(QIOChannel *ioc,
                                        size_t niov,
                                        int *fds,
                                        size_t nfds,
+                                       int flags,
                                        Error **errp)
 {
     QIOChannelFile *fioc = QIO_CHANNEL_FILE(ioc);
diff --git a/io/channel-socket.c b/io/channel-socket.c
index 459922c874..b2d254ef8d 100644
--- a/io/channel-socket.c
+++ b/io/channel-socket.c
@@ -525,6 +525,7 @@  static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
                                          size_t niov,
                                          int *fds,
                                          size_t nfds,
+                                         int flags,
                                          Error **errp)
 {
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
@@ -620,6 +621,7 @@  static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
                                          size_t niov,
                                          int *fds,
                                          size_t nfds,
+                                         int flags,
                                          Error **errp)
 {
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
diff --git a/io/channel-tls.c b/io/channel-tls.c
index 2ae1b92fc0..4ce890a538 100644
--- a/io/channel-tls.c
+++ b/io/channel-tls.c
@@ -301,6 +301,7 @@  static ssize_t qio_channel_tls_writev(QIOChannel *ioc,
                                       size_t niov,
                                       int *fds,
                                       size_t nfds,
+                                      int flags,
                                       Error **errp)
 {
     QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc);
diff --git a/io/channel-websock.c b/io/channel-websock.c
index 70889bb54d..035dd6075b 100644
--- a/io/channel-websock.c
+++ b/io/channel-websock.c
@@ -1127,6 +1127,7 @@  static ssize_t qio_channel_websock_writev(QIOChannel *ioc,
                                           size_t niov,
                                           int *fds,
                                           size_t nfds,
+                                          int flags,
                                           Error **errp)
 {
     QIOChannelWebsock *wioc = QIO_CHANNEL_WEBSOCK(ioc);
diff --git a/io/channel.c b/io/channel.c
index e8b019dc36..b8b99fdc4c 100644
--- a/io/channel.c
+++ b/io/channel.c
@@ -72,18 +72,32 @@  ssize_t qio_channel_writev_full(QIOChannel *ioc,
                                 size_t niov,
                                 int *fds,
                                 size_t nfds,
+                                int flags,
                                 Error **errp)
 {
     QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
 
-    if ((fds || nfds) &&
-        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
+    if (fds || nfds) {
+        if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_FD_PASS)) {
+            error_setg_errno(errp, EINVAL,
+                             "Channel does not support file descriptor passing");
+            return -1;
+        }
+        if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
+            error_setg_errno(errp, EINVAL,
+                             "Zero Copy does not support file descriptor passing");
+            return -1;
+        }
+    }
+
+    if ((flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) &&
+        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_WRITE_ZERO_COPY)) {
         error_setg_errno(errp, EINVAL,
-                         "Channel does not support file descriptor passing");
+                         "Requested Zero Copy feature is not available");
         return -1;
     }
 
-    return klass->io_writev(ioc, iov, niov, fds, nfds, errp);
+    return klass->io_writev(ioc, iov, niov, fds, nfds, flags, errp);
 }
 
 
@@ -217,14 +231,14 @@  int qio_channel_writev_all(QIOChannel *ioc,
                            size_t niov,
                            Error **errp)
 {
-    return qio_channel_writev_full_all(ioc, iov, niov, NULL, 0, errp);
+    return qio_channel_writev_full_all(ioc, iov, niov, NULL, 0, 0, errp);
 }
 
 int qio_channel_writev_full_all(QIOChannel *ioc,
                                 const struct iovec *iov,
                                 size_t niov,
                                 int *fds, size_t nfds,
-                                Error **errp)
+                                int flags, Error **errp)
 {
     int ret = -1;
     struct iovec *local_iov = g_new(struct iovec, niov);
@@ -235,10 +249,16 @@  int qio_channel_writev_full_all(QIOChannel *ioc,
                           iov, niov,
                           0, iov_size(iov, niov));
 
+    if (flags & QIO_CHANNEL_WRITE_FLAG_ZERO_COPY) {
+        assert(fds == NULL && nfds == 0);
+    }
+
     while (nlocal_iov > 0) {
         ssize_t len;
-        len = qio_channel_writev_full(ioc, local_iov, nlocal_iov, fds, nfds,
-                                      errp);
+
+        len = qio_channel_writev_full(ioc, local_iov, nlocal_iov, fds,
+                                            nfds, flags, errp);
+
         if (len == QIO_CHANNEL_ERR_BLOCK) {
             if (qemu_in_coroutine()) {
                 qio_channel_yield(ioc, G_IO_OUT);
@@ -277,7 +297,7 @@  ssize_t qio_channel_writev(QIOChannel *ioc,
                            size_t niov,
                            Error **errp)
 {
-    return qio_channel_writev_full(ioc, iov, niov, NULL, 0, errp);
+    return qio_channel_writev_full(ioc, iov, niov, NULL, 0, 0, errp);
 }
 
 
@@ -297,7 +317,7 @@  ssize_t qio_channel_write(QIOChannel *ioc,
                           Error **errp)
 {
     struct iovec iov = { .iov_base = (char *)buf, .iov_len = buflen };
-    return qio_channel_writev_full(ioc, &iov, 1, NULL, 0, errp);
+    return qio_channel_writev_full(ioc, &iov, 1, NULL, 0, 0, errp);
 }
 
 
@@ -473,6 +493,19 @@  off_t qio_channel_io_seek(QIOChannel *ioc,
     return klass->io_seek(ioc, offset, whence, errp);
 }
 
+int qio_channel_flush(QIOChannel *ioc,
+                                Error **errp)
+{
+    QIOChannelClass *klass = QIO_CHANNEL_GET_CLASS(ioc);
+
+    if (!klass->io_flush ||
+        !qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_WRITE_ZERO_COPY)) {
+        return 0;
+    }
+
+    return klass->io_flush(ioc, errp);
+}
+
 
 static void qio_channel_restart_read(void *opaque)
 {
diff --git a/migration/rdma.c b/migration/rdma.c
index c7c7a38487..ed2a2a013b 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -2833,6 +2833,7 @@  static ssize_t qio_channel_rdma_writev(QIOChannel *ioc,
                                        size_t niov,
                                        int *fds,
                                        size_t nfds,
+                                       int flags,
                                        Error **errp)
 {
     QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
diff --git a/scsi/pr-manager-helper.c b/scsi/pr-manager-helper.c
index 451c7631b7..3be52a98d5 100644
--- a/scsi/pr-manager-helper.c
+++ b/scsi/pr-manager-helper.c
@@ -77,7 +77,7 @@  static int pr_manager_helper_write(PRManagerHelper *pr_mgr,
         iov.iov_base = (void *)buf;
         iov.iov_len = sz;
         n_written = qio_channel_writev_full(QIO_CHANNEL(pr_mgr->ioc), &iov, 1,
-                                            nfds ? &fd : NULL, nfds, errp);
+                                            nfds ? &fd : NULL, nfds, 0, errp);
 
         if (n_written <= 0) {
             assert(n_written != QIO_CHANNEL_ERR_BLOCK);
diff --git a/tests/unit/test-io-channel-socket.c b/tests/unit/test-io-channel-socket.c
index c49eec1f03..6713886d02 100644
--- a/tests/unit/test-io-channel-socket.c
+++ b/tests/unit/test-io-channel-socket.c
@@ -444,6 +444,7 @@  static void test_io_channel_unix_fd_pass(void)
                             G_N_ELEMENTS(iosend),
                             fdsend,
                             G_N_ELEMENTS(fdsend),
+                            0,
                             &error_abort);
 
     qio_channel_readv_full(dst,