Date: Thu, 16 Apr 2009 19:28:03 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Yinghai Lu <yinghai@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Jesse Barnes <jbarnes@virtuousgeek.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       linux-pci@vger.kernel.org, yannick.roehlly@free.fr
Subject: Re: [PATCH] x86/pci: make pci_mem_start to be aligned only -v3
Message-ID: <20090416172803.GB16618@elte.hu>
References: <49E00E9F.8030605@kernel.org> <49E4F6D6.6030709@kernel.org> <49E4F71F.10107@kernel.org> <alpine.LFD.2.00.0904141359020.18124@localhost.localdomain> <49E52A7A.4070607@kernel.org> <49E52D3F.1090206@kernel.org> <20090416093152.6605612d@hobbes> <alpine.LFD.2.00.0904160938090.4042@localhost.localdomain> <20090416165640.GA13927@elte.hu> <49E76864.9060309@kernel.org>
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* Yinghai Lu <yinghai@kernel.org> wrote:

> Ingo Molnar wrote:
> > * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > 
> >> On Thu, 16 Apr 2009, Jesse Barnes wrote:
> >>> Any comments on this one, Linus?  Should I include your ack?
> >> I'm not ready to ack it, no. I don't think the suggested patch is very 
> >> clean or necessarily sensible as-is. It seems very ad-hoc. 
> >>
> >> I was literally thinking of something like 
> >>  "round up from the last RAM by X"
> >>  "round up from the last reserved region by Y"
> >>  "pick the bigger of the two"
> >>
> >> with helper functions for the two cases and comments along the 
> >> lines of why we do it. Something that was a bit more obvious about 
> >> what it's doing and why.
> > 
> > That's sensible - but i'd also like to inject hpa's add-on idea: if 
> > we do that then we should do it _explicitly_ and _visibly_, by 
> > injecting an artificial e820 reservation range to all expected 
> > "vulnerable" holes we cannot fully trust.
> > 
> > We'd do that after all the fixed resources are allocated, but before 
> > dynamic PCI allocations.
> > 
> > That prevents the PCI layer from dynamically allocating anything 
> > into that protective zone, and documents it as well (and makes it 
> > visible in boot logs, etc.) - instead of just a silent rule 
> > somewhere that no-one will really see if it breaks.
> 
> that need to do done much earlier, and much simple, just need to 
> make that range to be reserved in e820. and later e820_setup_gap 
> even don't need to be aligned again.

Well, an alignment _check_ could still be added with a
WARN_ONCE(), to make sure these assumptions hold true in
future as well.

This kind of stuff is generally not testable and wont break on many 
systems - but it can easily cripple a random 0.5% of systems, 
creating a lot of unhappy users.

So pretty much the only solution is to be careful, robust and 
redundant all along.

	Ingo
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