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

[Marc-André Lureau]

Hi

On Thu, May 31, 2018 at 9:46 AM, Sergio Lopez <address@hidden> wrote:
> 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) {

Instead of adding explicit handling of EPIPE, shouldn't the code be
rewritten to treat -1 return && errno != EAGAIN as fatal?

>              assert(s->watch_tag == 0);
>              s->watch_tag =
> --
> 1.8.3.1
>
>



-- 
Marc-André Lureau