linux-kernel - Re: [RFC PATCH 0/11] Support Write-Through mapping on x86

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Date:	Mon, 21 Jul 2014 10:31:38 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, akpm@...ux-foundation.org,
	arnd@...db.de, plagnioj@...osoft.com, tomi.valkeinen@...com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	stefan.bader@...onical.com, luto@...capital.net, airlied@...il.com,
	bp@...en8.de
Subject: Re: [RFC PATCH 0/11] Support Write-Through mapping on x86

On Wed, 2014-07-16 at 15:28 -0600, Toshi Kani wrote:
> On Tue, 2014-07-15 at 20:40 -0400, Konrad Rzeszutek Wilk wrote:
> > On July 15, 2014 5:23:24 PM EDT, Toshi Kani <toshi.kani@...com> wrote:
> > >On Tue, 2014-07-15 at 13:09 -0700, H. Peter Anvin wrote:
> > >> On 07/15/2014 12:34 PM, Toshi Kani wrote:
>  :
> > >> 
> > >> I have given this piece of feedback at least three times now,
> > >possibly
> > >> to different people, and I'm getting a bit grumpy about it:
> > >> 
> > >> We already have an issue with Xen, because Xen assigned mappings
> > >> differently and it is incompatible with the use of PAT in Linux.  As
> > >a
> > >> result we get requests for hacks to work around this, which is
> > >something
> > >> I really don't want to see.  I would like to see a design involving a
> > >> "reverse PAT" table where the kernel can hold the mapping between
> > >memory
> > >> types and page table encodings (including the two different ones for
> > >> small and large pages.)
> > >
> > >Thanks for pointing this out! (And sorry for making you repeat it three
> > >time...)  I was not aware of the issue with Xen.  I will look into the
> > >email archive to see what the Xen issue is, and how it can be
> > >addressed.
> > 
> > https://lkml.org/lkml/2011/11/8/406
> 
> Thanks Konrad for the pointer!
> 
> Since [__]change_page_attr_set_clr() and __change_page_attr() have no
> knowledge about PAT and simply work with specified PTE flags, they do
> not seem to fit well with additional PAT abstraction table...
> 
> I think the root of this issue is that the kernel ignores the PAT bit.
> Since __change_page_attr() only supports 4K pages, set_memory_<type>()
> can set the PAT bit into the clear mask.
> 
> Attached is a patch with this approach (apply on top of this series -
> not tested).  The kernel still does not support the PAT bit, but it
> behaves slightly better.

Hi Peter, Konrad,

Do you have any comments / suggestions for this approach?

Thanks!
-Toshi




View attachment "page-ext-mask.patch" of type "text/x-patch" (3717 bytes)