On Thu, 2014-07-10 at 12:34 +0100, Will Deacon wrote: > Hey Alex, > > On Wed, Jul 09, 2014 at 09:37:15PM +0100, Alex Williamson wrote: > > On Wed, 2014-07-09 at 19:28 +0100, Will Deacon wrote: > > > Hello, > > > > > > This is v2 of the RFC I originally posted here: > > > > > > RFCv1: http://permalink.gmane.org/gmane.linux.kernel.iommu/5552 > > > > > > It's changed significantly since then, based on helpful feedback from > > > Alex. In particular, I no longer butcher the IOMMU API, instead making > > > use of a new iommu_attr to indicate that a domain requires support for > > > nesting. > > > > > > All feedback welcome (this is still an RFC after all), > > > > It's a lot cleaner this way, but I'm concerned that allowing a "nesting" > > IOMMU to be created and used just like a TYPE1v2 IOMMU, even if the > > IOMMU doesn't support it, puts us in a corner for compatibility later. > > The safer approach might be to fail the attach_group function if we > > can't set the domain attribute, ie. enforce that the IOMMU must support > > it. > > I also contemplated this (and was my source of confusion when we were > discussing using iommu_attr before), however from a user's perspective it's > not at all clear what's gone wrong. Essentially, VFIO would advertise > support for VFIO_TYPE1_NESTING_IOMMU but any attempt to VFIO_SET_IOMMU > for a container would fail. Knowing to try and fall-back on VFIO_TYPE1v2_IOMMU > isn't at all obvious. > > > I don't know how you'd handle it if only some of the domains within > > a container supported it, so enforcement may simplify things down the > > road too. We wouldn't need the vfio_domains_have_iommu_nesting() > > function that way either. > > Agreed, it would certainly make things simpler in the kernel. Although, if > we allow a subset of domains in a container to use nesting, I think that's > possible by only allowing groups in those domains to be part of the virtual > SMMU interface. > > Do you think that returning something like -EOPNOTSUPP from an attach is > sufficient for userspace to figure out what's gone wrong?
It seems like this would all be a little easier if we had some sort of bus_type iterator, then when we want to see if nesting is supported we'd iterate the bus_types, test iommu_present(), iommu_domain_alloc(), iommu_domain_set_attr(), iommu_domain_free(). We'd only allow a nesting domain to be set if the IOMMU driver for at least one bus_type supports it. Then I think enforcing it would be a little more obvious to userspace. There is a bus_kset, but we'd need an iterator or at least to enable find_bus() and have type1 walk a list of bus names known to possibly support nesting (uglier, but functional). Thanks, Alex _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
