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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs
Date: Tue, 04 May 2010 11:25:28 -0500
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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...


Anthony Liguori


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