Message-ID: <1405964011.31163.1.camel@misato.fc.hp.com>
Subject: Re: [RFC PATCH 0/11] Support Write-Through mapping on x86
From: Toshi Kani <toshi.kani@hp.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, tglx@linutronix.de,
        mingo@redhat.com, akpm@linux-foundation.org, arnd@arndb.de,
        plagnioj@jcrosoft.com, tomi.valkeinen@ti.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, stefan.bader@canonical.com,
        luto@amacapital.net, airlied@gmail.com, bp@alien8.de
Date: Mon, 21 Jul 2014 11:33:31 -0600
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On Mon, 2014-07-21 at 10:32 -0700, H. Peter Anvin wrote:
> On 07/21/2014 10:16 AM, Toshi Kani wrote:
 :
> >> 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?
> 
> First of all, paravirt functions are the root of all evil, and we want
> to reduce and eliminate them to the utmost level possible.  But yes, we
> could plumb that up that way if we really need to.
> 
> What I'm thinking of is a table which can deal with both the moving PTE
> bit, Xen, and the scattered encodings by having a small table from types
> to encodings, and not use the encodings directly until fairly late it
> the pipe.  I suspect, but I'm not sure, that we would also need the
> inverse operation.

Thanks for the explanation!  I will think about it as well.
-Toshi

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