On Fri, Jun 18, 2021 at 04:57:40PM +0000, Tian, Kevin wrote: > > From: Jason Gunthorpe <jgg@xxxxxxxxxx> > > Sent: Friday, June 18, 2021 8:20 AM > > > > On Thu, Jun 17, 2021 at 03:14:52PM -0600, Alex Williamson wrote: > > > > > I've referred to this as a limitation of type1, that we can't put > > > devices within the same group into different address spaces, such as > > > behind separate vRoot-Ports in a vIOMMU config, but really, who cares? > > > As isolation support improves we see fewer multi-device groups, this > > > scenario becomes the exception. Buy better hardware to use the devices > > > independently. > > > > This is basically my thinking too, but my conclusion is that we should > > not continue to make groups central to the API. > > > > As I've explained to David this is actually causing functional > > problems and mess - and I don't see a clean way to keep groups central > > but still have the device in control of what is happening. We need > > this device <-> iommu connection to be direct to robustly model all > > the things that are in the RFC. > > > > To keep groups central someone needs to sketch out how to solve > > today's mdev SW page table and mdev PASID issues in a clean > > way. Device centric is my suggestion on how to make it clean, but I > > haven't heard an alternative?? > > > > So, I view the purpose of this discussion to scope out what a > > device-centric world looks like and then if we can securely fit in the > > legacy non-isolated world on top of that clean future oriented > > API. Then decide if it is work worth doing or not. > > > > To my mind it looks like it is not so bad, granted not every detail is > > clear, and no code has be sketched, but I don't see a big scary > > blocker emerging. An extra ioctl or two, some special logic that > > activates for >1 device groups that looks a lot like VFIO's current > > logic.. > > > > At some level I would be perfectly fine if we made the group FD part > > of the API for >1 device groups - except that complexifies every user > > space implementation to deal with that. It doesn't feel like a good > > trade off. > > > > Would it be an acceptable tradeoff by leaving >1 device groups > supported only via legacy VFIO (which is anyway kept for backward > compatibility), if we think such scenario is being deprecated over > time (thus little value to add new features on it)? Then all new > sub-systems including vdpa and new vfio only support singleton > device group via /dev/iommu... The case that worries me here is if you *thought* you had 1 device groups, but then discover a hardware bug which means two things aren't as isolated as you thought they were. What do you do then? -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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