linux-kernel - Re: [PATCH V4 1/3] x86: Make page cache mode a real type

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Date:	Wed, 29 Oct 2014 11:36:05 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Juergen Gross <jgross@...e.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, hpa@...or.com,
	x86@...nel.org, mingo@...hat.com, stefan.bader@...onical.com,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, ville.syrjala@...ux.intel.com,
	david.vrabel@...rix.com, jbeulich@...e.com, toshi.kani@...com
Subject: Re: [PATCH V4 1/3] x86: Make page cache mode a real type


* Juergen Gross <jgross@...e.com> wrote:

> On 10/29/2014 11:28 AM, Ingo Molnar wrote:
> >
> >* Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> >>On Mon, 27 Oct 2014, Juergen Gross wrote:
> >>>At the moment there are a lot of places that handle setting or getting
> >>>the page cache mode by treating the pgprot bits equal to the cache mode.
> >>>This is only true because there are a lot of assumptions about the setup
> >>>of the PAT MSR. Otherwise the cache type needs to get translated into
> >>>pgprot bits and vice versa.
> >>>
> >>>This patch tries to prepare for that by introducing a separate type
> >>>for the cache mode and adding functions to translate between those and
> >>>pgprot values.
> >>>
> >>>To avoid too much performance penalty the translation between cache mode
> >>>and pgprot values is done via tables which contain the relevant
> >>>information.  Write-back cache mode is hard-wired to be 0, all other
> >>>modes are configurable via those tables. For large pages there are
> >>>translation functions as the PAT bit is located at different positions
> >>>in the ptes of 4k and large pages.
> >>
> >>I'm fine with the approach itself. Though I wish you had split this
> >>patch in several sanely to review pieces.
> >>
> >>   - Introduce enums, helper functions etc. which basically reflect
> >>     the state of today
> >>
> >>   - Change the usage sites to use the enums and helpers
> >>
> >>   - Convert the enum/helper implementation to the new scheme
> >>
> >>   - Split out the printk change
> >>
> >>   ...
> >
> >That absolutely has to be done, should any of this introduce
> >regressions. The diffstat:
> >
> >  18 files changed, 344 insertions(+), 212 deletions(-)
> >
> >is _way_ too large.
> 
> Okay, will do.

Thanks!

	Ingo
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