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

[Toshi Kani]

On Tue, 2014-07-15 at 13:09 -0700, H. Peter Anvin wrote:
> On 07/15/2014 12:34 PM, Toshi Kani wrote:
> > This RFC patchset is aimed to seek comments/suggestions for the design
> > and changes to support of Write-Through (WT) mapping.  The study below
> > shows that using WT mapping may be useful for non-volatile memory.
> > 
> >   http://www.hpl.hp.com/techreports/2012/HPL-2012-236.pdf
> > 
> > There were idea & patches to support WT in the past, which stimulated
> > very valuable discussions on this topic.
> > 
> >   https://lkml.org/lkml/2013/4/24/424
> >   https://lkml.org/lkml/2013/10/27/70
> >   https://lkml.org/lkml/2013/11/3/72
> > 
> > This RFC patchset tries to address the issues raised by taking the
> > following design approach:
> > 
> >  - Keep the MTRR interface
> >  - Keep the WB, WC, and UC- slots in the PAT MSR
> >  - Keep the PAT bit unused
> >  - Reassign the UC slot to WT in the PAT MSR
> > 
> > There are 4 usable slots in the PAT MSR, which are currently assigned to:
> > 
> >   PA0/4: WB, PA1/5: WC, PA2/6: UC-, PA3/7: UC
> > 
> > The PAT bit is unused since it shares the same bit as the PSE bit and
> > there was a bug in older processors.  Among the 4 slots, the uncached
> > memory type consumes 2 slots, UC- and UC.  They are functionally
> > equivalent, but UC- allows MTRRs to overwrite it with WC.  All interfaces
> > that set the uncached memory type use UC- in order to work with MTRRs.
> > The PA3/7 slot is effectively unused today.  Therefore, this patchset
> > reassigns the PA3/7 slot to WT.  If MTRRs get deprecated in future,
> > UC- can be reassigned to UC, and there is still no need to consume
> > 2 slots for the uncached memory type.
> 
> Not going to happen any time in the forseeable future.
> 
> Furthermore, I don't think it is a big deal if on some old, buggy
> processors we take the performance hit of cache type demotion, as long
> as we don't actively lose data.
> 
> > This patchset is consist of two parts.  The 1st part, patch [1/11] to
> > [6/11], enables WT mapping and adds new interfaces for setting WT mapping.
> > The 2nd part, patch [7/11] to [11/11], cleans up the code that has
> > internal knowledge of the PAT slot assignment.  This keeps the kernel
> > code independent from the PAT slot assignment.
> 
> 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.

Thanks,
-Toshi


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