[Qemu-devel] [Bug 1586756] Re: "-serial unix:" option of qemu-system-* is broken in qemu 2.6.0

[redcap97]

I wrote small code which reproduces this issue.

https://bitbucket.org/redcap97/puts-hello-x80000-armv7-kernel/downloads
/puts-HELLO-x80000

Above binary outputs "HELLO!" 80000 times to UART.

# Please execute in terminal
socat unix-listen:a.sock stdout | tee actual

# Please execute in another terminal
qemu-system-arm -M vexpress-a9 -nographic -kernel puts-HELLO-x80000 -serial 
unix:a.sock

# Check results
yes 'HELLO!' | head -n 80000 > expected
diff -u expected actual

Occasionally, a part of the output of qemu disappears.


Source code of the binary
https://bitbucket.org/redcap97/puts-hello-x80000-armv7-kernel/src

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https://bugs.launchpad.net/bugs/1586756

Title:
  "-serial unix:" option of qemu-system-* is broken in qemu 2.6.0

Status in QEMU:
  Incomplete

Bug description:
  I found a bug of "-serial unix:PATH_TO_SOCKET" in qemu 2.6.0 (qemu 2.5.1 
works fine).
  Occasionally, a part of the output of qemu disappears in the bug.

  It looks like following commit is the cause:

  char: ensure all clients are in non-blocking mode (Author: Daniel P. Berrange 
<address@hidden>)
  
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=64c800f808748522727847b9cdc73412f22dffb9

  In this commit, UNIX socket is set to non-blocking mode, but 
qemu_chr_fe_write function doesn't handle EAGAIN.
  You should fix code like that:

  ---
  diff --git a/qemu-char.c b/qemu-char.c
  index b597ee1..0361d78 100644
  --- a/qemu-char.c
  +++ b/qemu-char.c
  @@ -270,6 +270,7 @@ static int qemu_chr_fe_write_buffer(CharDriverState *s, 
const uint8_t *buf, int
   int qemu_chr_fe_write(CharDriverState *s, const uint8_t *buf, int len)
   {
       int ret;
  +    int offset = 0;
   
       if (s->replay && replay_mode == REPLAY_MODE_PLAY) {
           int offset;
  @@ -280,7 +281,21 @@ int qemu_chr_fe_write(CharDriverState *s, const uint8_t 
*buf, int len)
       }
   
       qemu_mutex_lock(&s->chr_write_lock);
  -    ret = s->chr_write(s, buf, len);
  +
  +    while (offset < len) {
  +    retry:
  +        ret = s->chr_write(s, buf, len);
  +        if (ret < 0 && errno == EAGAIN) {
  +            g_usleep(100);
  +            goto retry;
  +        }
  +
  +        if (ret <= 0) {
  +            break;
  +        }
  +
  +        offset += ret;
  +    }
   
       if (ret > 0) {
           qemu_chr_fe_write_log(s, buf, ret);
  ---

  Or please do "git revert 64c800f808748522727847b9cdc73412f22dffb9".

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