> From: Jason Gunthorpe <jgg@xxxxxxxxxx> > Sent: Wednesday, May 5, 2021 1:13 AM > > On Wed, Apr 28, 2021 at 06:58:19AM +0000, Tian, Kevin wrote: > > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > > Sent: Wednesday, April 28, 2021 1:12 AM > > > > > [...] > > > > As Alex says, if this line fails because of the group restrictions, > > > > that's not great because it's not very obvious what's gone wrong. > > > > > > Okay, that is fair, but let's solve that problem directly. For > > > instance netlink has been going in the direction of adding a "extack" > > > from the kernel which is a descriptive error string. If the failing > > > ioctl returned the string: > > > > > > "cannot join this device to the IOASID because device XXX in the > > > same group #10 is in use" > > > > > > Would you agree it is now obvious what has gone wrong? In fact would > > > you agree this is a lot better user experience than what applications > > > do today even though they have the group FD? > > > > > > > Currently all the discussions are around implicit vs. explicit uAPI semantics > > on the group restriction. However if we look beyond group the implicit > > semantics might be inevitable when dealing with incompatible iommu > > domains. An existing example of iommu incompatibility is IOMMU_ > > CACHE. > > I still think we need to get rid of these incompatibilities > somehow. Having multiple HW incompatible IOASID in the same platform > is just bad all around. > > When modeling in userspace IOMMU_CACHE sounds like it is a property of > each individual IOASID, not an attribute that requires a new domain. sure. the iommu domain is an kernel-internal concept. The userspace should focus everything on IOASID. > > People that want to create cache bypass IOASID's should just ask for > that that directly. > Yes, in earlier discussion we agreed on a scheme that ioasid module will return an error to userspace indicating incompatibility detected when binding a device to ioasid then the userspace should create a new IOASID for this device. This has to be done 'explicitly'. When I used it as the example for 'implicit semantics" is that the kernel won't create another group-like object to contain devices with compatible attributes and 'explicitly' manage it in uAPI like group_fd. If we anyway rely on the userspace to have more intelligence on those hardware restrictions, it's little sense to only explicitly handle group_fd in uAPI. Thanks Kevin
