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

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 5 Jun 2018 11:18:35 +0200
Paolo Bonzini <pbonzini@redhat.com> 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 <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 =
> >   
Hi Sergio, Paolo,

it looks like commit 
commit 019288bf137183bf3407c9824655b753bfafc99f
Author: Sergio Lopez <slp@redhat.com>
Date:   Tue Jun 5 03:54:55 2018 -0400

    hw/char/serial: Only retry if qemu_chr_fe_write returns 0

introduced regression wrt 2.12 and broke windows guest remote kernel debug over
serial, where windbg can't connect to target and target hangs when windbg tries
to connect to it.

Reverting the commit fixes issue