linux-kernel - Re: [PATCH] VMI: remove CONFIG_DEBUG_PAGE

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Date:	Fri, 06 Jul 2007 14:01:45 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Chris Wright <chrisw@...s-sol.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization@...ts.osdl.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	"Robert P. J. Day" <rpjday@...dspring.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Carsten Otte <cotte@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] VMI: remove CONFIG_DEBUG_PAGE_TYPE and associated	bitrotted
 code

Zachary Amsden wrote:
> I though about it, but it gets really ugly.  You need wrappers for all 
> the MMU ops in pvops generic code, which means either another layer of 
> wrappers or a bunch of CONFIG_DEBUG_PARAVIRT

Oh, yes, more wrappers please!  We could do it at the paravirt_ops 
level: set up your pv_ops, then call paravirt_debug_mmuops(), which 
would save away your ops and replace them with wrappers.  That basic 
structure would lend itself to all kinds of paravirt-level debugging tools.

It would be a bit more elegant if we had mmu_ops.  Maybe we should do 
that splitup before 64bit?

> only things that are easy to break because they also depend on PAE vs. 
> non-PAE.

Hm, would they?  Would they need to inspect the content of the pte_t 
(etc), or just look at the struct page for the page being updated?  (pmd 
operations being a bit more awkward, of course.)

> It's doable, though, and might even be extensible to s390 for CMM page 
> type debugging, as well as descriptor type tracking and enforcement of 
> page isolation of GDTs.
>
> Page state tracking could track -
>
> PAGE_ZERO, PAGE_UNUSED, PAGE_STABLE, PAGE_VOLATILE, 
> PAGE_POTENTIALLY_VOLATILE, PAGE_L1{2/3/4}, PAGE_LDT, PAGE_GDT,
>
> actually, no this seems silly, since we'd just be duplicating bits for 
> the page types, so the only debug benefit is ensuring the intersection 
> of volatile and L{1/2/3/4} is nil, which is already trivially 
> verifiable by inspection. 

Well, I have to say that keeping the hypervisor hints in sync with the 
actual kernel-level page state worries me, so any debug tools which 
could help there would be good.  This seems like it should be the right 
place to do it, but I can't say I've thought about it in any detail.

Of course, if it *is* helpful to the page hinting patches, then it 
suggests that it's a facility with wider scope than pv-ops.

    J
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