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

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Message-ID: <1405962993.30151.35.camel@misato.fc.hp.com>
Date:	Mon, 21 Jul 2014 11:16:33 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, 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 Mon, 2014-07-21 at 09:47 -0700, H. Peter Anvin wrote:
> On 07/21/2014 09:31 AM, Toshi Kani wrote:
> > Do you have any comments / suggestions for this approach?
> 
> Approach to what, specifically?
>
> Keep in mind the PAT bit is different for large pages.  This needs to be
> dealt with.  

You are right.  I was under a wrong impression that
__change_page_attr() always splits a large pages into 4KB pages, but I
overlooked the fact that it can handle a large page as well.  So, this
approach does not work...

> I would also like a systematic way to deal with the fact
> that Xen (sigh) is stuck with a separate mapping system.
>
> I guess Linux could adopt the Xen mappings if that makes it easier, as
> long as that doesn't have a negative impact on native hardware -- we can
> possibly deal with some older chips not being optimal.  

I see.  I agree that supporting the PAT bit is the right direction, but
I do not know how much effort we need.  I will study on this.

> However, my thinking has been to have a "reverse PAT" table in memory of memory
> types to encodings, both for regular and large pages.

I am not clear about your idea of the "reverse PAT" table.  Would you
care to elaborate?  How is it different from using pte_val() being a
paravirt function on Xen?

Thanks,
-Toshi

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