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

[Anthony Liguori]

On 05/04/2010 11:01 AM, Alexander Graf wrote:
> Am 04.05.2010 um 16:34 schrieb Anthony Liguori <address@hidden>:
>
> > 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.
>
> The problem is that the select list of fds includes the stdio fd, so 
> that gets notified and is coupled with virtio-console, but there's 
> nothing passing that on to mux and I don't think it'd be clever to 
> expose internal data to the muxer to tell it about the backend fds.

When stdio is readable, it should invoke qemu_chr_read() with the read 
data which in turn ought to invoke mux_chr_read().

I'm not sure I understand what signalling is missing.  Jan, does the 
problem Alex describes ring a bell?  I seem to recall you saying that 
mux was still fundamentally broken but ought to work most of the time...

Regards,

Anthony Liguori

> Alex