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

[Toshi Kani]

On Mon, 2014-07-21 at 14:33 -0400, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 21, 2014 at 10:32:34AM -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
> 
> Here I was thinking to actually put an entry in the MAINTAINERS
> file for me to become the owner of it - as the folks listed there
> are busy with other things.
> 
> The Maintainer of 'All Evil' has an interesting ring to it :-)

:-)

> > 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.
> 
> Mr Toshi-san,

Oh, you are so polite, Wilk-san. 

> This link: http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/hvm/mtrr.c;h=ee18553cdac58dd16836011ee714517fbc16368d;hb=HEAD#l74 might help you in figuring how this can be done.
> 
> Thought I have to say that the code is quite complex so it might
> be more confusing then helpful.

Thanks again for the pointer!  I will take a look.  I used to work on a
paravirt on other OS, but I am pretty much new to Xen.  One more thing
to learn. :-)
-Toshi



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