linux-kernel - Re: hardwired VMI crap

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Message-ID: <45F0A254.4020002@vmware.com>
Date:	Thu, 08 Mar 2007 15:55:00 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	john stultz <johnstul@...ibm.com>, akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pratap Subrahmanyam <pratap@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	Daniel Hecht <dhecht@...are.com>,
	Daniel Arai <arai@...are.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>
Subject: Re: hardwired VMI crap

Jeremy Fitzhardinge wrote:
> No, but I'm not prejudiced against virtual hardware.  If we have a piece
> of code that thinks its talking to an apic, then I think its OK to use
> that code whether its a real apic or a virtual one, _so long as its
> being used in a way that's consistent with its intended interface_.  I
> have to admit I have not looked at apics - real or virtual - in any
> detail, so I won't claim to really understand the details of the
> existing arch/i386 code or what VMI's trying to do, but it does seem to
> me that it could all be much cleaner.
>
> And clean is good, we all love clean - and so, agreement!
>   

For APICs, we have two operations - APICRead and APICWrite.  It is nice 
and clean, and plugs in very easily to the APIC accessors available in 
Linux.

Is this not clean?

We just don't drive the local timer interrupts through the APIC, we make 
hypercalls to schedule local timer alarms.  Which is something we must 
do for UP kernels as well, which use the PIT / PIC.  So there is a need 
for having clockevents code which doesn't program timers through the APIC.

So we have one separate time device, independent from the traditional 
hardware timers, and we just program that.  This design is not very 
complex, nor is it unclean, IMHO.

Zach
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