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 22:57:22 +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 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.

Well, why not option D):

   D) detect unaligned requests and reject them

?

Thanks,

	Ingo
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