Re: [Qemu-devel] [PATCH 3/5] sclpconsole: remove bogus check for -EAGAIN

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed,  3 Aug 2016 17:22:38 +0100
"Daniel P. Berrange" <berrange@redhat.com> wrote:

s/sclpconsole/sclpconsolelm/ in the subject, as there are two sclp
consoles :)

> The write_console_data() method in sclpconsole-lm.c checks
> whether the return value of qemu_chr_fe_write() has the
> value of -EAGAIN and if so then increments the buffer offset
> by the value of EAGAIN. Fortunately qemu_chr_fe_write() will
> never return EAGAIN directly, rather it returns -1 with
> errno set to EAGAIN, so this broken code path was not
> reachable. The behaviour on EAGAIN was stil bad though,
> causing the write_console_data() to busy_wait repeatedly
> calling qemu_chr_fe_write() with no sleep between iters.
> 
> Just remove all this loop logic and replace with a call
> to qemu_chr_fe_write_all().
> 
> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> ---
>  hw/char/sclpconsole-lm.c | 18 +++---------------
>  1 file changed, 3 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/char/sclpconsole-lm.c b/hw/char/sclpconsole-lm.c
> index a22ad8d..8cb0026 100644
> --- a/hw/char/sclpconsole-lm.c
> +++ b/hw/char/sclpconsole-lm.c
> @@ -201,21 +201,9 @@ static int write_console_data(SCLPEvent *event, const uint8_t *buf, int len)
>          return len;
>      }
> 
> -    buf_offset = buf;
> -    while (len > 0) {
> -        ret = qemu_chr_fe_write(scon->chr, buf, len);
> -        if (ret == 0) {
> -            /* a pty doesn't seem to be connected - no error */
> -            len = 0;
> -        } else if (ret == -EAGAIN || (ret > 0 && ret < len)) {
> -            len -= ret;
> -            buf_offset += ret;
> -        } else {
> -            len = 0;
> -        }
> -    }
> -
> -    return ret;
> +    /* XXX this blocks entire thread. Rewrite to use
> +     * qemu_chr_fe_write and background I/O callbacks */
> +    return qemu_chr_fe_write_all(scon->chr, buf, len);

This is basically the same change that had been done for the sclp vt220
console already in 2e14211 ("s390/sclpconsole: handle char layer busy
conditions"), so this looks fine.

>  }
> 
>  static int process_mdb(SCLPEvent *event, MDBO *mdbo)

Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>