linux-kernel - Re: [patch 0/3] kvm tool: Serial emulation overhaul

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Date:	Sun, 11 Dec 2011 15:04:37 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <levinsasha928@...il.com>
Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul

On Sun, 11 Dec 2011, Ingo Molnar wrote:
> In theory a kvm driven serial console should be extremely fast, 
> much faster than a real serial console, basically as fast as a 
> local console. Yet this still does not seem to be the case.
> 
> In fact even ssh-ing in to a box over Wifi and running the above 
> top session is undistinguishable from top running in a local 
> console. So IMO it cannot be virtualization overhead - there 
> must still be some delay or serious lack of buffering somewhere.

Well, the difference between ssh and serial is, that ssh can pack 1.5k
worth of data into one frame, while serial has to send it
piecewise. And the emulation has to trap into kvm tool for each tx
byte, which doesnt help either. We cannot do much with buffering on
the kvm tool side as we have no clue how much consecutive data will
come in. That's why there is a virtual console, which has the
disadvantage that you cant see the early boot messages.

Thanks,

	tglx

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