Re: [Qemu-devel] [PATCH] hw/char/serial: Only retry if qemu_chr_fe_write returns 0

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/06/2018 09:54, Sergio Lopez wrote:
> Only retry on serial_xmit if qemu_chr_fe_write returns 0, as this is the
> only recoverable error.
> 
> Retrying with any other scenario, in addition to being a waste of CPU
> cycles, can compromise the Guest stability if by the vCPU issuing the
> write and the main loop thread are, by chance or explicit pinning,
> running on the same pCPU.
> 
> Previous discussion:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg06998.html
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  hw/char/serial.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/char/serial.c b/hw/char/serial.c
> index 605b0d0..6de6c29 100644
> --- a/hw/char/serial.c
> +++ b/hw/char/serial.c
> @@ -260,7 +260,7 @@ static void serial_xmit(SerialState *s)
>          if (s->mcr & UART_MCR_LOOP) {
>              /* 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 &&
> +        } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) == 0 &&
>                     s->tsr_retry < MAX_XMIT_RETRY) {
>              assert(s->watch_tag == 0);
>              s->watch_tag =
> 

Queued, thanks to you and all those who participated in the discussion.

Paolo