linux-kernel - Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86

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Message-ID: <20150218211555.GA22696@gmail.com>
Date:	Wed, 18 Feb 2015 22:15:55 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Toshi Kani <toshi.kani@...com>
Cc:	akpm@...ux-foundation.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, arnd@...db.de, linux-mm@...ck.org,
	x86@...nel.org, linux-kernel@...r.kernel.org, Elliott@...com
Subject: Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86


* Toshi Kani <toshi.kani@...com> wrote:

> On Wed, 2015-02-18 at 21:44 +0100, Ingo Molnar wrote:
> > * Toshi Kani <toshi.kani@...com> wrote:
> > 
> > > This patch implements huge I/O mapping capability interfaces on x86.
> > 
> > > +#ifdef CONFIG_HUGE_IOMAP
> > > +#ifdef CONFIG_X86_64
> > > +#define IOREMAP_MAX_ORDER       (PUD_SHIFT)
> > > +#else
> > > +#define IOREMAP_MAX_ORDER       (PMD_SHIFT)
> > > +#endif
> > > +#endif  /* CONFIG_HUGE_IOMAP */
> > 
> > > +#ifdef CONFIG_HUGE_IOMAP
> > 
> > Hm, so why is there a Kconfig option for this? It just 
> > complicates things.
> > 
> > For example the kernel already defaults to mapping itself 
> > with as large mappings as possible, without a Kconfig entry 
> > for it. There's no reason to make this configurable - and 
> > quite a bit of complexity in the patches comes from this 
> > configurability.
> 
> This Kconfig option was added to disable this feature in 
> case there is an issue. [...]

If bugs are found then they should be fixed.

> [...]  That said, since the patchset also added a new 
> nohugeiomap boot option for the same purpose, I agree 
> that this Kconfig option can be removed.  So, I will 
> remove it in the next version.
> 
> An example of such case is with multiple MTRRs described 
> in patch 0/7.

So the multi-MTRR case should probably be detected and 
handled safely?

Thanks,

	Ingo
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