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

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Date:	Mon, 12 Dec 2011 18:45:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...ux.intel.com>
Subject: Re: [patch 0/3] kvm tool: Serial emulation overhaul


* Sasha Levin <levinsasha928@...il.com> wrote:

> On Mon, 2011-12-12 at 12:12 +0100, Ingo Molnar wrote:
> > * Sasha Levin <levinsasha928@...il.com> wrote:
> > 
> > > 50k cycles for every single byte is pretty much as good as it 
> > > will get with serial console. See slide #5 in Marcelo's KVM 
> > > Forum 2010 presentation[1] where he timed a heavyweight exit 
> > > to about 40k cycles.
> > 
> > > [1]
> > > http://www.linux-kvm.org/wiki/images/e/ea/2010-forum-mtosatti_walkthrough_entry_exit.pdf
> > 
> > But what we do here is a PIO exit. That, according to 
> > Marcelo's measurements, is about 10K cycles, back to back. 
> > [*]

> No, everything on slide 5 is part of a PIO exit. Starting from 
> 'out' and up to until it's back to the guest. A total of over 
> 40k cycles.

No, slide 5 is a cumulative total, with the '+' showing the 
individual costs, adding up to 10400 cycles total.

Please read that slide again ;-)

10K is bad enough IMO, 40K cycles would be 'out of this world 
crazy'.

Thanks,

	Ingo
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