From: Marc-André Lureau <marcandre.lur...@redhat.com>

As Ed Swierk explained back in 2006:
https://lists.nongnu.org/archive/html/qemu-devel/2006-12/msg00160.html

"When qemu writes into the pipe, it immediately reads back what it just
wrote and treats it as a monitor command, endlessly breathing its own
exhaust."

This is similarly confusing when using the chardev with a serial device,
as reported in https://bugzilla.redhat.com/show_bug.cgi?id=2106975.

It seems we have kept the support for bidirectional pipes for historical
reasons and odd systems, however it's not documented in qemu -chardev
options. I suggest to stop supporting it, for portability reasons.

Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com>
---
 docs/about/deprecated.rst | 6 ++++++
 chardev/char-pipe.c       | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
index 7ee26626d5cf..dd5ca30d527b 100644
--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -45,6 +45,12 @@ transmit audio through the VNC protocol.
 ``tty`` and ``parport`` are aliases that will be removed. Instead, the
 actual backend names ``serial`` and ``parallel`` should be used.
 
+``-chardev pipe`` support for bidirectional pipes (since 7.1)
+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+For portability reasons, the support for bidirectional ``pipe`` will
+be removed. Instead, use ``file.in`` & ``file.out`` unidirectional pipes.
+
 Short-form boolean options (since 6.0)
 ''''''''''''''''''''''''''''''''''''''
 
diff --git a/chardev/char-pipe.c b/chardev/char-pipe.c
index 66d3b8509183..7db963035e7d 100644
--- a/chardev/char-pipe.c
+++ b/chardev/char-pipe.c
@@ -27,6 +27,7 @@
 #include "qemu/main-loop.h"
 #include "qemu/module.h"
 #include "qemu/option.h"
+#include "qemu/error-report.h"
 #include "chardev/char.h"
 
 #ifdef _WIN32
@@ -142,6 +143,9 @@ static void qemu_chr_open_pipe(Chardev *chr,
         if (fd_out >= 0) {
             close(fd_out);
         }
+        warn_report("Support for bidirectional pipe is deprecated,");
+        warn_report("please use portable one-way pipes instead (%s.in & 
%s.out).",
+                    filename, filename);
         TFR(fd_in = fd_out = qemu_open_old(filename, O_RDWR | O_BINARY));
         if (fd_in < 0) {
             error_setg_file_open(errp, errno, filename);
-- 
2.37.0.rc0

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