On 06.11.2019 18:31, Andrew Cooper wrote: > On 06/11/2019 15:16, Jan Beulich wrote: >> update_paging_mode() in the AMD IOMMU code expects to be invoked with >> the PCI devices lock held. The check occurring only when the mode >> actually needs updating, the violation of this rule by the majority >> of callers did go unnoticed until per-domain IOMMU setup was changed >> to do away with on-demand creation of IOMMU page tables. >> >> Unfortunately the only half way reasonable fix to this that I could >> come up with requires more re-work than would seem desirable at this >> time of the release process, but addressing the issue seems >> unavoidable to me as its manifestation is a regression from the >> IOMMU page table setup re-work. The change also isn't without risk >> of further regressions - if in patch 2 I've missed a code path that >> would also need to invoke the new hook, then this might mean non- >> working guests (with passed-through devices on AMD hardware). >> >> 1: AMD/IOMMU: don't needlessly trigger errors/crashes when unmapping a page >> 2: introduce GFN notification for translated domains >> 3: AMD/IOMMU: use notify_dfn() hook to update paging mode > > Having now looked at all three, why don't we just delete the dynamic > height of AMD IOMMU pagetables? > > This series looks suspiciously like it is adding new common > infrastructure to work around the fact we're doing something fairly dumb > to being with. > > Hardcoding at 4 levels is, at the very worst, 2 extra pages per domain, > and a substantial reduction in the complexity of the IOMMU code.
Yet an additional level of page walks hardware has to perform. Also 4 levels won't cover all possible 52 address bits. And finally, the more applicable your "substantial reduction", the less suitable such a change may be at this point of the release (but I didn't look at this side of things in any detail, so it may well not be an issue). Jan _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
