On Fri, 2022-05-06 at 06:49 +0000, Tian, Kevin wrote: > > From: Baolu Lu <[email protected]> > > > > > --- 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. > 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). 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? > another nit: dmar is intel specific thus CONFIG_X86 is always true. DMAR exists on IA64 too.
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