linux-kernel - Re: ACPI early ioremap problems

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Message-ID: <20080119153055.GA12054@elte.hu>
Date:	Sat, 19 Jan 2008 16:30:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: ACPI early ioremap problems

(Cc:-ing to lkml, because this might interest others too)

* Andi Kleen <andi@...stfloor.org> wrote:

> > would be interesting to figure out what's going on here - i doubt 
> > it's an ACPI bug, because then we'd be getting a hard page fault, 
> > right? In
> 
> ACPI hasn't changed at all in git-x86 and it worked fine before. So I 
> don't really blame it.

yes, it did not change, but there are latent ACPI bugs/uncleanlinesses 
where it references an already unmapped table. This worked by chance 
until now, because we didnt actually unmap any tables - but now we 
explicitly map/unmap the tables via the MMU. But, i dont think that's 
the cause of the failure here - those bugs typically show up in other 
ways.

> > that case it's a 64-bit early_ioremap() bug that we want to find 
> > even if ACPI didnt use early_ioremap().
> > 
> > and this all runs before zap_low_mappings(), right?
> 
> No after. Since some time x86-64 does the equivalent of z_l_m() in 
> head64(); this means before start_kernel and definitely before 
> setup_arch which sets up ACPI.

that would mean early_ioremap() should switch to ioremap() after that 
point. Could you try that, does it resolve the failure you are seeing? 

Long-term we want to have a single, uniform ioremap() interface (on 
32-bit and 64-bit x86 as well) that can be used anytime, which just 
switches to the right lowlevel method depending on how far we are into 
the pagetable and memory subsystem bootstrap - instead of these more 
fragile "can we now use early_ioremap() or should we already be using 
ioremap()" usages.

	Ingo
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