On Thu, Jul 16, 2020 at 10:38:17PM +0200, Auger Eric wrote: > Hi Jean, > > On 7/16/20 5:39 PM, Jean-Philippe Brucker wrote: > > On Tue, Jul 14, 2020 at 10:12:49AM +0000, Liu, Yi L wrote: > >>> Have you verified that this doesn't break the existing usage of > >>> DOMAIN_ATTR_NESTING in drivers/vfio/vfio_iommu_type1.c? > >> > >> I didn't have ARM machine on my hand. But I contacted with Jean > >> Philippe, he confirmed no compiling issue. I didn't see any code > >> getting DOMAIN_ATTR_NESTING attr in current drivers/vfio/vfio_iommu_type1.c. > >> What I'm adding is to call iommu_domai_get_attr(, DOMAIN_ATTR_NESTIN) > >> and won't fail if the iommu_domai_get_attr() returns 0. This patch > >> returns an empty nesting info for DOMAIN_ATTR_NESTIN and return > >> value is 0 if no error. So I guess it won't fail nesting for ARM. > > > > I confirm that this series doesn't break the current support for > > VFIO_IOMMU_TYPE1_NESTING with an SMMUv3. That said... > > > > If the SMMU does not support stage-2 then there is a change in behavior > > (untested): after the domain is silently switched to stage-1 by the SMMU > > driver, VFIO will now query nesting info and obtain -ENODEV. Instead of > > succeding as before, the VFIO ioctl will now fail. I believe that's a fix > > rather than a regression, it should have been like this since the > > beginning. No known userspace has been using VFIO_IOMMU_TYPE1_NESTING so > > far, so I don't think it should be a concern. > But as Yi mentioned ealier, in the current vfio code there is no > DOMAIN_ATTR_NESTING query yet. That's why something that would have succeeded before will now fail: Before this series, if user asked for a VFIO_IOMMU_TYPE1_NESTING, it would have succeeded even if the SMMU didn't support stage-2, as the driver would have silently fallen back on stage-1 mappings (which work exactly the same as stage-2-only since there was no nesting supported). After the series, we do check for DOMAIN_ATTR_NESTING so if user asks for VFIO_IOMMU_TYPE1_NESTING and the SMMU doesn't support stage-2, the ioctl fails. I believe it's a good fix and completely harmless, but wanted to make sure no one objects because it's an ABI change. Thanks, Jean > In my SMMUV3 nested stage series, I added > such a query in vfio-pci.c to detect if I need to expose a fault region > but I already test both the returned value and the output arg. So to me > there is no issue with that change. > > > > And if userspace queries the nesting properties using the new ABI > > introduced in this patchset, it will obtain an empty struct. I think > > that's acceptable, but it may be better to avoid adding the nesting cap if > > @format is 0? > agreed > > Thanks > > Eric > > > > Thanks, > > Jean > > > >> > >> @Eric, how about your opinion? your dual-stage vSMMU support may > >> also share the vfio_iommu_type1.c code. > >> > >> Regards, > >> Yi Liu > >> > >>> Will > > >
