> From: Steve Wahl <steve.w...@hpe.com> > Sent: Friday, May 6, 2022 11:26 PM > > On Fri, May 06, 2022 at 08:12:11AM +0000, Tian, Kevin wrote: > > > From: David Woodhouse <dw...@infradead.org> > > > Sent: Friday, May 6, 2022 3:17 PM > > > > > > On Fri, 2022-05-06 at 06:49 +0000, Tian, Kevin wrote: > > > > > From: Baolu Lu <baolu...@linux.intel.com> > > > > > > > > > > > --- a/include/linux/dmar.h > > > > > > +++ b/include/linux/dmar.h > > > > > > @@ -19,7 +19,7 @@ > > > > > > struct acpi_dmar_header; > > > > > > > > > > > > #ifdef CONFIG_X86 > > > > > > -# define DMAR_UNITS_SUPPORTED MAX_IO_APICS > > > > > > +# define DMAR_UNITS_SUPPORTED 640 > > > > > > #else > > > > > > # define DMAR_UNITS_SUPPORTED 64 > > > > > > #endif > > > > > > > > ... is it necessary to permanently do 10x increase which wastes memory > > > > on most platforms which won't have such need. > > > > > > I was just looking at that. It mostly adds about 3½ KiB to each struct > > > dmar_domain. > > > > > > I think the only actual static array is the dmar_seq_ids bitmap which > > > grows to 640 *bits* which is fairly negligible, and the main growth is > > > that it adds about 3½ KiB to each struct dmar_domain for the > > > iommu_refcnt[] and iommu_did[] arrays. > > > > Thanks for the quick experiment! though the added material is > > negligible it's cleaner to me if having a way to configure it as > > discussed below. > > > > > > > > > Does it make more sense to have a configurable approach similar to > > > > CONFIG_NR_CPUS? or even better can we just replace those static > > > > arrays with dynamic allocation so removing this restriction > > > > completely? > > > > > > Hotplug makes that fun, but I suppose you only need to grow the array > > > in a given struct dmar_domain if you actually add a device to it that's > > > behind a newly added IOMMU. I don't know if the complexity of making it > > > fully dynamic is worth it though. We could make it a config option, > > > and/or a command line option (perhaps automatically derived from > > > CONFIG_NR_CPUS). > > > > either config option or command line option is OK to me. Probably > > the former is simpler given no need to dynamically expand the > > static array. btw though deriving from CONFIG_NR_CPUS could work > > in this case it is unclear why tying the two together is necessary in > > concept, e.g. is there guarantee that the number of IOMMUs must > > be smaller than the number of CPUs in a platform? > > > > > > > > If it wasn't for hotplug, I think we'd know the right number by the > > > time we actually need it anyway, wouldn't we? Can we have a heuristic > > > for how many DMAR units are likely to be hotplugged? Is it as simple as > > > the ratio of present to not-yet-present CPUs in MADT? > > > > Probably. But I don't have enough knowledge on DMAR hotplug to > > judge (e.g. whether it's strictly tied to CPU hotplug and if yes whether > > there could be multiple IOMMUs hotplugged together with a CPU > > socket)... > > > > Thanks > > Kevin > > Would anyone be more comfortable if we only increase the limit where > MAXSMP is set? > > i.e. > > #if defined(CONFIG_X86) && defined(CONFIG_MAXSMP) > # define DMAR_UNITS_SUPPORTED 640 > #elif defined(CONFIG_X86) > # define DMAR_UNITS_SUPPORTED MAX_IO_APICS > #else > # define DMAR_UNITS_SUPPORTED 64 > #endif > > Thank you all for your time looking at this. >
This works for your own configuration but it's unclear whether other MAXSMP platforms have the exact same requirements (different number of sockets, different ratio of #iommus/#sockets, etc.). In any case since we are at it having a generic way to extend it makes more sense to me. Thanks Kevin _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
