On Thu, Oct 29, 2015 at 06:42:10PM +0000, Robin Murphy wrote: > On 29/10/15 18:28, Will Deacon wrote: > >On Tue, Oct 27, 2015 at 10:00:11AM -0600, Alex Williamson wrote: > >>On Tue, 2015-10-27 at 15:40 +0000, Will Deacon wrote: > >>>On Fri, Oct 16, 2015 at 09:51:22AM -0600, Alex Williamson wrote: > >>>>Would it be possible to add iommu_domain_geometry support to arm-smmu.c? > >>>>In addition to this test to verify that DMA cannot bypass the IOMMU, I'd > >>>>eventually like to pass the aperture information out through the VFIO > >>>>API. Thanks, > >>> > >>>The slight snag here is that we don't know which SMMU in the system the > >>>domain is attached to at the point when the geometry is queried, so I > >>>can't give you an upper bound on the aperture. For example, if there is > >>>an SMMU with a 32-bit input address and another with a 48-bit input > >>>address. > >>> > >>>We could play the same horrible games that we do with the pgsize bitmap, > >>>and truncate some global aperture everytime we probe an SMMU device, but > >>>I'd really like to have fewer hacks like that if possible. The root of > >>>the problem is still that domains are allocated for a bus, rather than > >>>an IOMMU instance. > >> > >>Yes, Intel VT-d has this issue as well. In theory we can have > >>heterogeneous IOMMU hardware units (DRHDs) in a system and the upper > >>bound of the geometry could be diminished if we add a less capable DRHD > >>into the domain. I suspect this is more a theoretical problem than a > >>practical one though as we're typically mixing similar DRHDs and I think > >>we're still capable of 39-bit addressing in the least capable version > >>per the spec. > >> > >>In any case, I really want to start testing geometry.force_aperture, > >>even if we're not yet comfortable to expose the IOMMU limits to the > >>user. The vfio type1 shouldn't be enabled at all for underlying > >>hardware that allows DMA bypass. Thanks, > > > >Ok, I'll put it on my list of things to look at under the assumption that > >the actual aperture limits don't need to be accurate as long as DMA to > >an arbitrary unmapped address always faults. > > I'm pretty sure we'd only ever set the aperture to the full input address > range anyway (since we're not a GART-type thing), in which case we should > only need to worry about unmatched streams that don't hit in a domain at > all. Doesn't the disable_bypass option already cover that? (FWIW I hacked it > up for v2 a while back, too[0]). Well, the "full input address range" is tricky when you have multiple SMMU instances with different input address sizes. I can do something similar to the pgsize_bitmap. I also don't think the disable_bypass option is what we're after -- this is about devices attached to a VFIO domain that can still mysteriously bypass the SMMU for some ranges AFAIU (and shouldn't be an issue for ARM). Will -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
