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

[Marc-André Lureau]

Hi

On Tue, Jun 5, 2018 at 11:18 AM, Paolo Bonzini <address@hidden> wrote:
> 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 <address@hidden>
>> ---
>>  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.

Shouldn't it rety if it returns -1 and the errno == EAGAIN? I am not
sure when it would return 0 and a retry would make sense.



-- 
Marc-André Lureau