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

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH 3/6] Make char muxer more robust wrt small FIFOs
Date: Wed, 07 Apr 2010 16:32:04 +0200
User-agent: Thunderbird (X11/20090817)

Amit Shah wrote:
> 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.

Probably, yes.

>> 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?

Hrm - would it actually matter? We need to have older guest kernels
working anyways. And my S390 LPAR doesn't exactly have a lot of disk
space, so compiling a kernel is anything but fun :-).


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