On Fri, Feb 21, 2020 at 11:24:56AM -0700, Logan Gunthorpe wrote: > Hi, > > This is v3 of the patchset which cleans up a number of minor issues > from the feedback of v2 and rebases onto v5.6-rc2. Additional feedback > is welcome. > > Thanks, > > Logan > > -- > > Changes in v3: > * Rebased onto v5.6-rc2 > * Rename mhp_modifiers to mhp_params per David with an updated kernel > doc per Dan > * Drop support for s390 per David seeing it does not support > ZONE_DEVICE yet and there was a potential problem with huge pages. > * Added WARN_ON_ONCE in cases where arches recieve non PAGE_KERNEL > parameters > * Collected David and Micheal's Reviewed-By and Acked-by Tags > > Changes in v2: > * Rebased onto v5.5-rc5 > * Renamed mhp_restrictions to mhp_modifiers and added the pgprot field > to that structure instead of using an argument for > arch_add_memory(). > * Add patch to drop the unused flags field in mhp_restrictions > > A git branch is available here: > > https://github.com/sbates130272/linux-p2pmem remap_pages_cache_v3 > > -- > > Currently, the page tables created using memremap_pages() are always > created with the PAGE_KERNEL cacheing mode. However, the P2PDMA code > is creating pages for PCI BAR memory which should never be accessed > through the cache and instead use either WC or UC. This still works in > most cases, on x86, because the MTRR registers typically override the > caching settings in the page tables for all of the IO memory to be > UC-. However, this tends not to work so well on other arches or > some rare x86 machines that have firmware which does not setup the > MTRR registers in this way. > > Instead of this, this series proposes a change to arch_add_memory() > to take the pgprot required by the mapping which allows us to > explicitly set pagetable entries for P2PDMA memory to WC. Is there a particular reason why WC was selected here? I thought for the p2pdma cases there was no kernel user that touched the memory? I definitely forsee devices where we want UC instead. Even so, the whole idea looks like the right direction to me. Jason
