[Qemu-devel] [PATCH 3/3] hw/char/serial: Don't retry on serial_xmit if errno == EPIPE

[Sergio Lopez <slp@redhat.com>]

If writing to the frontend channel failed with EPIPE, don't set up a
retry. EPIPE is not a recoverable error, so trying again is a waste of CPU
cycles.

If the vCPU writing to the serial device and emulator thread are pinned
to the same pCPU, it can also compromise the stability of the Guest OS,
as both threads will be competing for pCPU's time, with the vCPU
actively polling the serial device and barely giving time to the
emulator thread to make actual progress.
---
 hw/char/serial.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index 2c080c9..f26e86b 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -262,6 +262,7 @@ static void serial_xmit(SerialState *s)
             /* in loopback mode, say that we just received a char */
             serial_receive1(s, &s->tsr, 1);
         } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) != 1 &&
+                   errno != EPIPE &&
                    s->tsr_retry < MAX_XMIT_RETRY) {
             assert(s->watch_tag == 0);
             s->watch_tag =
-- 
1.8.3.1