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

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Date:	Wed, 18 Feb 2015 14:33:29 -0700
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
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

On Wed, 2015-02-18 at 22:15 +0100, Ingo Molnar wrote:
> * 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.

Right.

> > [...]  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?

I considered two options to safely handle this case, i.e. option A) and
B) described in the link below.
  https://lkml.org/lkml/2015/2/5/638

I thought about how much complication we should put into the code for an
imaginable platform with a combination of new NVM (or large I/O range)
and legacy MTRRs with multi-types & contiguous ranges.  My thinking is
that we should go with option C) for simplicity, and implement A) or B)
later if we find it necessary.

Thanks,
-Toshi





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