On 25/04/2019 11:01, Jan Beulich wrote: >>>> On 23.04.19 at 20:36, <jgr...@suse.com> wrote: >> On 23/04/2019 19:05, Stefano Stabellini wrote: >>> On Tue, 23 Apr 2019, Juergen Gross wrote: >>>> Instead of always calling xen_destroy_contiguous_region() in case the >>>> memory is DMA-able for the used device, do so only in case it has been >>>> made DMA-able via xen_create_contiguous_region() before. >>>> >>>> This will avoid a lot of xen_destroy_contiguous_region() calls for >>>> 64-bit capable devices. >>>> >>>> As the memory in question is owned by swiotlb-xen the PG_owner_priv_1 >>>> flag of the first allocated page can be used for remembering. >>> >>> Although the patch looks OK, this sentence puzzles me. Why do you say >>> that the memory in question is owned by swiotlb-xen? Because it was >>> returned by xen_alloc_coherent_pages? Both the x86 and the Arm >>> implementation return fresh new memory, hence, it should be safe to set >>> the PageOwnerPriv1 flag? >>> >>> My concern with this approach is with the semantics of PG_owner_priv_1. >>> Is a page marked with PG_owner_priv_1 only supposed to be used by the >>> owner? >> >> The owner of the page is free to use the flag. >> >> Like Grant pages are marked by the grant driver using this flag. And >> Xen page tables are using it in PV-guests for indicating a "Pinned" >> page table. > > Considering the background of the series, isn't such multi-purpose use > of the flag a possible problem? You're already suspecting a wrong call > into here. The function finding the flag set (but for another reason) > might add to the confusion. But I realize there are only so many page > flags available.
Right. > Perhaps the freeing function should, first thing, check the handed > space actually matches the criteria (within dma_mask and contiguous)? Yes, I think this is a good idea. Juergen _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
