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.) -hpa -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>