Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs

[Anthony Liguori]

On 05/04/2010 09:30 AM, Alexander Graf wrote:
> Am 04.05.2010 um 15:44 schrieb Anthony Liguori <address@hidden>:
>
> > On 04/20/2010 11:56 AM, Alexander Graf wrote:
> > > Virtio-Console can only process one character at a time. Using it on 
> > > S390
> > > gave me strage "lags" where I got the character I pressed before when
> > > pressing one. So I typed in "abc" and only received "a", then 
> > > pressed "d"
> > > but the guest received "b" and so on.
> > >
> > > While the stdio driver calls a poll function that just processes on its
> > > queue in case virtio-console can't take multiple characters at once, 
> > > the
> > > muxer does not have such callbacks, so it can't empty its queue.
> > >
> > > To work around that limitation, I introduced a new timer that only gets
> > > active when the guest can not receive any more characters. In that case
> > > it polls again after a while to check if the guest is now receiving 
> > > input.
> > >
> > > This patch fixes input when using -nographic on s390 for me.
> >
> > I think this is really a kvm issue.  I assume it's because s390 idles 
> > in the kernel so you never drop to userspace to repoll the descriptor.
>
> There is no polling for the muxer. That's why it never knows when 
> virtio-console can receive again.

Maybe I'm missing something simple, but it looks to me like the muxer is 
polling.  mux_chr_can_read() is going to eventually poll the muxed 
devices to figure this out.

If the root of the problem is that mux_chr_can_read() isn't being 
invoked for a prolonged period of time, the real issue is the problem I 
described.

Regards,

Anthony Liguori

> This patch basically adfs timer based polling for that exact case.
>
>
> Alex
>
> > A timer is a hacky solution.  You really need to use an io thread to 
> > solve this and then you need to switch away from qemu_set_fd_handler2 
> > to qemu_set_fd_handler() and make sure that the later breaks select 
> > whenever it's invoked.
> >
> > Regards,
> >
> > Anthony Liguori
> >
> > > ---
> > >
> > > Please consider for stable.
> > > ---
> > >  qemu-char.c |   10 ++++++++++
> > >  1 files changed, 10 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/qemu-char.c b/qemu-char.c
> > > index 05df971..ce9df3a 100644
> > > --- a/qemu-char.c
> > > +++ b/qemu-char.c
> > > @@ -235,6 +235,7 @@ typedef struct {
> > >      IOEventHandler *chr_event[MAX_MUX];
> > >      void *ext_opaque[MAX_MUX];
> > >      CharDriverState *drv;
> > > +    QEMUTimer *accept_timer;
> > >      int focus;
> > >      int mux_cnt;
> > >      int term_got_escape;
> > > @@ -396,6 +397,13 @@ static void 
> > > mux_chr_accept_input(CharDriverState *chr)
> > >          d->chr_read[m](d->ext_opaque[m],
> > > &d->buffer[m][d->cons[m]++&  MUX_BUFFER_MASK], 1);
> > >      }
> > > +
> > > +    /* We're still not able to sync producer and consumer, so let's 
> > > wait a bit
> > > +       and try again by then. */
> > > +    if (d->prod[m] != d->cons[m]) {
> > > +        qemu_mod_timer(d->accept_timer, qemu_get_clock(vm_clock)
> > > +                                        + (int64_t)100000);
> > > +    }
> > >  }
> > >
> > >  static int mux_chr_can_read(void *opaque)
> > > @@ -478,6 +486,8 @@ static CharDriverState 
> > > *qemu_chr_open_mux(CharDriverState *drv)
> > >      chr->opaque = d;
> > >      d->drv = drv;
> > >      d->focus = -1;
> > > +    d->accept_timer = qemu_new_timer(vm_clock,
> > > +                                     
> > > (QEMUTimerCB*)mux_chr_accept_input, chr);
> > >      chr->chr_write = mux_chr_write;
> > >      chr->chr_update_read_handler = mux_chr_update_read_handler;
> > >      chr->chr_accept_input = mux_chr_accept_input;