Re: [Qemu-devel] [PATCH 3/6] Make char muxer more robust wrt small FIFOs

[Amit Shah]

On (Thu) Apr 01 2010 [18:42:38], Alexander Graf wrote:
> Virtio-Console can only process one character at a time.

The host can process as many as you give it, depending on the buffer
size exposed by the guest.

On older guests (guest kernels w/o multiport support), the guest reads
input from host, processes it and only then opens up another buffer for
the host to write into.

On newer guests (guest kernels that support multiport), the guest
fills the entire vq so that host can send as many buffers as possible
without getting throttled. I guess you're getting hit by this.

> 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.

This might be because qemu-char would not be able to send out 'b' while
the guest still processes 'a' and has no free buffers to write out to.
On seeing 'd', it flushes its queue.

Can you try using a 2.6.34-rc3 kernel without this patch to see if
things work fine?

                Amit