Date: Wed, 18 Feb 2015 22:15:55 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Toshi Kani <toshi.kani@hp.com>
Cc: akpm@linux-foundation.org, hpa@zytor.com, tglx@linutronix.de,
        mingo@redhat.com, arnd@arndb.de, linux-mm@kvack.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Elliott@hp.com
Subject: Re: [PATCH v2 6/7] x86, mm: Support huge I/O mappings on x86
Message-ID: <20150218211555.GA22696@gmail.com>
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 <20150218204414.GA20943@gmail.com>
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* Toshi Kani <toshi.kani@hp.com> wrote:

> On Wed, 2015-02-18 at 21:44 +0100, Ingo Molnar wrote:
> > * Toshi Kani <toshi.kani@hp.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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