linux-kernel - Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

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Date:	Thu, 08 Mar 2007 21:26:54 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, tglx@...utronix.de,
	Dan Hecht <dhecht@...are.com>,
	Zachary Amsden <zach@...are.com>, akpm@...ux-foundation.org,
	ak@...e.de,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	LKML <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>,
	James Morris <jmorris@...ei.org>
Subject: Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

On Thu, 2007-03-08 at 09:01 +0100, Ingo Molnar wrote:
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > > Your implementation is almost the perfect prototype, if you move the 
> > > 128 bit hackery into the hypervisor and hide it away from the kernel 
> > > :)
> > 
> > The point is to use the tsc to avoid making any hypercalls, so dealing 
> > with the tsc->ns conversion has to happen on the guest side somehow.
> 
> you are obsessed with avoiding a hypercall, but why? Granted it's slow 
> especially on things like SVN/VMX, but it's not fundamentally slow. We 
> definitely do not want to design our whole APIs and abstractions around 
> the temporary notion that 'hypercalls are slow'. I'd expect hypercalls 
> to be put into silicon just as much as SYSENTER was put into silicon. 

Indeed, I expect them to fall somewhere between system calls and context
switches.  Perhaps not slow, but definitely worth minimising.

> Anyway, in terms of guest time code, a /big/ amount of design junk can 
> be avoided by not trying to do sillynesses like 'virtual time'. The TSC 
> is awfully unreliable.

You mean stolen time?

I find this whole discussion really irritating, to be honest.  I just
want Thomas to implement the timer code for lguest, because that code
scares me...

I look forward to your patch 8)
Rusty.


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