linux-kernel - Re: hardwired VMI crap

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Message-ID: <45F0A5F0.7040900@goop.org>
Date:	Thu, 08 Mar 2007 16:10:24 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
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

Zachary Amsden wrote:
> 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?

Sure, that's clean, From that perspective the apic is a bunch of
registers backed by a state machine or something.  It's not particularly
clean from a high-level interface perspective because those calls don't
mean anything, but that just means pv_ops is the wrong interface for
those calls. genapic, from its name alone, sounds like it should be the
right place to hook in at that level; if it isn't, it sounds like the
right starting place.

But...

> 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.

Yes, but couldn't you, oh I don't know, have the virtual timer
interrupts come in on irq 97, and just register a handler for that irq
and use that ISR to drive the time stuff?  Then its logically identical
to the Xen code or any other free-standing device driver.

Making your virtual timer device share interrupts with the (emulated)
real-time device seems to be making things messy (is that right, is that
the issue?).  I don't see why UP vs SMP is an issue here at all, or why
the PIT gets involved in any way (and I don't mean that in a "I think
your design is idiotic" way, I mean that in a "I don't really understand
the problem domain, so I'm missing something in your explanations" way).

    J
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